diff --git a/res/css/_components.scss b/res/css/_components.scss
index ad3cfbdcea7..657d77974ff 100644
--- a/res/css/_components.scss
+++ b/res/css/_components.scss
@@ -75,6 +75,7 @@
 @import "./views/dialogs/_InviteDialog.scss";
 @import "./views/dialogs/_KeyboardShortcutsDialog.scss";
 @import "./views/dialogs/_MessageEditHistoryDialog.scss";
+@import "./views/dialogs/_ModalWidgetDialog.scss";
 @import "./views/dialogs/_NewSessionReviewDialog.scss";
 @import "./views/dialogs/_RoomSettingsDialog.scss";
 @import "./views/dialogs/_RoomSettingsDialogBridges.scss";
diff --git a/res/css/views/dialogs/_ModalWidgetDialog.scss b/res/css/views/dialogs/_ModalWidgetDialog.scss
new file mode 100644
index 00000000000..aa2dd0d3958
--- /dev/null
+++ b/res/css/views/dialogs/_ModalWidgetDialog.scss
@@ -0,0 +1,42 @@
+/*
+Copyright 2020 The Matrix.org Foundation C.I.C.
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+*/
+
+.mx_ModalWidgetDialog {
+    .mx_ModalWidgetDialog_warning {
+        margin-bottom: 24px;
+
+        > img {
+            vertical-align: middle;
+            margin-right: 8px;
+        }
+    }
+
+    .mx_ModalWidgetDialog_buttons {
+        float: right;
+        margin-top: 24px;
+
+        .mx_AccessibleButton + .mx_AccessibleButton {
+            margin-left: 8px;
+        }
+    }
+
+    iframe {
+        width: 100%;
+        height: 450px;
+        border: 0;
+        border-radius: 8px;
+    }
+}
diff --git a/res/css/views/elements/_AccessibleButton.scss b/res/css/views/elements/_AccessibleButton.scss
index 96269cea433..9c26f8f1206 100644
--- a/res/css/views/elements/_AccessibleButton.scss
+++ b/res/css/views/elements/_AccessibleButton.scss
@@ -25,7 +25,7 @@ limitations under the License.
 .mx_AccessibleButton_hasKind {
     padding: 7px 18px;
     text-align: center;
-    border-radius: 4px;
+    border-radius: 8px;
     display: inline-block;
     font-size: $font-14px;
 }
diff --git a/res/img/element-icons/warning-badge.svg b/res/img/element-icons/warning-badge.svg
new file mode 100644
index 00000000000..ac5991f2212
--- /dev/null
+++ b/res/img/element-icons/warning-badge.svg
@@ -0,0 +1,5 @@
+<svg width="16" height="16" viewBox="0 0 16 16" fill="none" xmlns="http://www.w3.org/2000/svg">
+<circle cx="8" cy="8" r="8" fill="#737D8C" style="mix-blend-mode:multiply"/>
+<rect x="7" y="3" width="2" height="6" rx="1" fill="white"/>
+<rect x="7" y="11" width="2" height="2" rx="1" fill="white"/>
+</svg>
diff --git a/src/@types/global.d.ts b/src/@types/global.d.ts
index ed28a5c4791..161aa3797f7 100644
--- a/src/@types/global.d.ts
+++ b/src/@types/global.d.ts
@@ -34,6 +34,7 @@ import WidgetStore from "../stores/WidgetStore";
 import CallHandler from "../CallHandler";
 import {Analytics} from "../Analytics";
 import UserActivity from "../UserActivity";
+import {ModalWidgetStore} from "../stores/ModalWidgetStore";
 
 declare global {
     interface Window {
@@ -60,6 +61,7 @@ declare global {
         mxCallHandler: CallHandler;
         mxAnalytics: Analytics;
         mxUserActivity: UserActivity;
+        mxModalWidgetStore: ModalWidgetStore;
     }
 
     interface Document {
diff --git a/src/Modal.tsx b/src/Modal.tsx
index b0f6ef988e5..2f761e73937 100644
--- a/src/Modal.tsx
+++ b/src/Modal.tsx
@@ -28,7 +28,7 @@ import AsyncWrapper from './AsyncWrapper';
 const DIALOG_CONTAINER_ID = "mx_Dialog_Container";
 const STATIC_DIALOG_CONTAINER_ID = "mx_Dialog_StaticContainer";
 
-interface IModal<T extends any[]> {
+export interface IModal<T extends any[]> {
     elem: React.ReactNode;
     className?: string;
     beforeClosePromise?: Promise<boolean>;
@@ -38,7 +38,7 @@ interface IModal<T extends any[]> {
     close(...args: T): void;
 }
 
-interface IHandle<T extends any[]> {
+export interface IHandle<T extends any[]> {
     finished: Promise<T>;
     close(...args: T): void;
 }
diff --git a/src/components/views/dialogs/ModalWidgetDialog.tsx b/src/components/views/dialogs/ModalWidgetDialog.tsx
new file mode 100644
index 00000000000..6ce3230a7aa
--- /dev/null
+++ b/src/components/views/dialogs/ModalWidgetDialog.tsx
@@ -0,0 +1,165 @@
+/*
+Copyright 2020 The Matrix.org Foundation C.I.C.
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+*/
+
+import * as React from 'react';
+import BaseDialog from './BaseDialog';
+import { _t } from '../../../languageHandler';
+import AccessibleButton from "../elements/AccessibleButton";
+import {
+    ClientWidgetApi,
+    IModalWidgetCloseRequest,
+    IModalWidgetOpenRequestData,
+    IModalWidgetReturnData,
+    ModalButtonKind,
+    Widget,
+    WidgetApiFromWidgetAction,
+} from "matrix-widget-api";
+import {StopGapWidgetDriver} from "../../../stores/widgets/StopGapWidgetDriver";
+import {MatrixClientPeg} from "../../../MatrixClientPeg";
+import RoomViewStore from "../../../stores/RoomViewStore";
+import {OwnProfileStore} from "../../../stores/OwnProfileStore";
+
+interface IProps {
+    widgetDefinition: IModalWidgetOpenRequestData;
+    sourceWidgetId: string;
+    onFinished(success: boolean, data?: IModalWidgetReturnData): void;
+}
+
+interface IState {
+    messaging?: ClientWidgetApi;
+}
+
+const MAX_BUTTONS = 3;
+
+export default class ModalWidgetDialog extends React.PureComponent<IProps, IState> {
+    private readonly widget: Widget;
+    private appFrame: React.RefObject<HTMLIFrameElement> = React.createRef();
+
+    state: IState = {};
+
+    constructor(props) {
+        super(props);
+
+        this.widget = new Widget({
+            ...this.props.widgetDefinition,
+            creatorUserId: MatrixClientPeg.get().getUserId(),
+            id: `modal_${this.props.sourceWidgetId}`,
+        });
+    }
+
+    public componentDidMount() {
+        const driver = new StopGapWidgetDriver( []);
+        const messaging = new ClientWidgetApi(this.widget, this.appFrame.current, driver);
+        this.setState({messaging});
+    }
+
+    public componentWillUnmount() {
+        this.state.messaging.off("ready", this.onReady);
+        this.state.messaging.off(`action:${WidgetApiFromWidgetAction.CloseModalWidget}`, this.onWidgetClose);
+        this.state.messaging.stop();
+    }
+
+    private onReady = () => {
+        this.state.messaging.sendWidgetConfig(this.props.widgetDefinition);
+    };
+
+    private onLoad = () => {
+        this.state.messaging.once("ready", this.onReady);
+        this.state.messaging.on(`action:${WidgetApiFromWidgetAction.CloseModalWidget}`, this.onWidgetClose);
+    };
+
+    private onWidgetClose = (ev: CustomEvent<IModalWidgetCloseRequest>) => {
+        this.props.onFinished(true, ev.detail.data);
+    }
+
+    public render() {
+        const templated = this.widget.getCompleteUrl({
+            currentRoomId: RoomViewStore.getRoomId(),
+            currentUserId: MatrixClientPeg.get().getUserId(),
+            userDisplayName: OwnProfileStore.instance.displayName,
+            userHttpAvatarUrl: OwnProfileStore.instance.getHttpAvatarUrl(),
+        });
+
+        const parsed = new URL(templated);
+
+        // Add in some legacy support sprinkles (for non-popout widgets)
+        // TODO: Replace these with proper widget params
+        // See https://github.com/matrix-org/matrix-doc/pull/1958/files#r405714833
+        parsed.searchParams.set('widgetId', this.widget.id);
+        parsed.searchParams.set('parentUrl', window.location.href.split('#', 2)[0]);
+
+        // Replace the encoded dollar signs back to dollar signs. They have no special meaning
+        // in HTTP, but URL parsers encode them anyways.
+        const widgetUrl = parsed.toString().replace(/%24/g, '$');
+
+        let buttons;
+        if (this.props.widgetDefinition.buttons) {
+            // show first button rightmost for a more natural specification
+            buttons = this.props.widgetDefinition.buttons.slice(0, MAX_BUTTONS).reverse().map(def => {
+                let kind = "secondary";
+                switch (def.kind) {
+                    case ModalButtonKind.Primary:
+                        kind = "primary";
+                        break;
+                    case ModalButtonKind.Secondary:
+                        kind = "primary_outline";
+                        break
+                    case ModalButtonKind.Danger:
+                        kind = "danger";
+                        break;
+                }
+
+                const onClick = () => {
+                    this.state.messaging.notifyModalWidgetButtonClicked(def.id);
+                };
+
+                return <AccessibleButton key={def.id} kind={kind} onClick={onClick}>
+                    { def.label }
+                </AccessibleButton>;
+            });
+        }
+
+        return <BaseDialog
+            title={this.props.widgetDefinition.name || _t("Modal Widget")}
+            className="mx_ModalWidgetDialog"
+            contentId="mx_Dialog_content"
+            onFinished={this.props.onFinished}
+        >
+            <div className="mx_ModalWidgetDialog_warning">
+                <img
+                    src={require("../../../../res/img/element-icons/warning-badge.svg")}
+                    height="16"
+                    width="16"
+                    alt=""
+                />
+                {_t("Data on this screen is shared with %(widgetDomain)s", {
+                    widgetDomain: parsed.hostname,
+                })}
+            </div>
+            <div>
+                <iframe
+                    ref={this.appFrame}
+                    sandbox="allow-forms allow-scripts allow-same-origin"
+                    src={widgetUrl}
+                    onLoad={this.onLoad}
+                />
+            </div>
+            <div className="mx_ModalWidgetDialog_buttons">
+                { buttons }
+            </div>
+        </BaseDialog>;
+    }
+}
diff --git a/src/i18n/strings/en_EN.json b/src/i18n/strings/en_EN.json
index 400fa247131..1548dd5c139 100644
--- a/src/i18n/strings/en_EN.json
+++ b/src/i18n/strings/en_EN.json
@@ -1760,6 +1760,8 @@
     "Verify session": "Verify session",
     "Your homeserver doesn't seem to support this feature.": "Your homeserver doesn't seem to support this feature.",
     "Message edits": "Message edits",
+    "Modal Widget": "Modal Widget",
+    "Data on this screen is shared with %(widgetDomain)s": "Data on this screen is shared with %(widgetDomain)s",
     "Your account is not secure": "Your account is not secure",
     "Your password": "Your password",
     "This session, or the other session": "This session, or the other session",
diff --git a/src/stores/ModalWidgetStore.ts b/src/stores/ModalWidgetStore.ts
new file mode 100644
index 00000000000..0485afd1062
--- /dev/null
+++ b/src/stores/ModalWidgetStore.ts
@@ -0,0 +1,87 @@
+/*
+Copyright 2020 The Matrix.org Foundation C.I.C.
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+*/
+
+import { AsyncStoreWithClient } from "./AsyncStoreWithClient";
+import defaultDispatcher from "../dispatcher/dispatcher";
+import { ActionPayload } from "../dispatcher/payloads";
+import Modal, {IHandle, IModal} from "../Modal";
+import ModalWidgetDialog from "../components/views/dialogs/ModalWidgetDialog";
+import {WidgetMessagingStore} from "./widgets/WidgetMessagingStore";
+import {IModalWidgetOpenRequestData, IModalWidgetReturnData, Widget} from "matrix-widget-api";
+
+interface IState {
+    modal?: IModal<any>;
+    openedFromId?: string;
+}
+
+export class ModalWidgetStore extends AsyncStoreWithClient<IState> {
+    private static internalInstance = new ModalWidgetStore();
+    private modalInstance: IHandle<void[]> = null;
+    private openSourceWidgetId: string = null;
+
+    private constructor() {
+        super(defaultDispatcher, {});
+    }
+
+    public static get instance(): ModalWidgetStore {
+        return ModalWidgetStore.internalInstance;
+    }
+
+    protected async onAction(payload: ActionPayload): Promise<any> {
+        // nothing
+    }
+
+    public canOpenModalWidget = () => {
+        return !this.modalInstance;
+    };
+
+    public openModalWidget = (requestData: IModalWidgetOpenRequestData, sourceWidget: Widget) => {
+        if (this.modalInstance) return;
+        this.openSourceWidgetId = sourceWidget.id;
+        this.modalInstance = Modal.createTrackedDialog('Modal Widget', '', ModalWidgetDialog, {
+            widgetDefinition: {...requestData},
+            sourceWidgetId: sourceWidget.id,
+            onFinished: (success: boolean, data?: IModalWidgetReturnData) => {
+                if (!success) {
+                    this.closeModalWidget(sourceWidget, { "m.exited": true });
+                } else {
+                    this.closeModalWidget(sourceWidget, data);
+                }
+
+                this.openSourceWidgetId = null;
+                this.modalInstance = null;
+            },
+        });
+    };
+
+    public closeModalWidget = (sourceWidget: Widget, data?: IModalWidgetReturnData) => {
+        if (!this.modalInstance) return;
+        if (this.openSourceWidgetId === sourceWidget.id) {
+            this.openSourceWidgetId = null;
+            this.modalInstance.close();
+            this.modalInstance = null;
+
+            const sourceMessaging = WidgetMessagingStore.instance.getMessaging(sourceWidget);
+            if (!sourceMessaging) {
+                console.error("No source widget messaging for modal widget");
+                return;
+            }
+            sourceMessaging.notifyModalWidgetClose(data);
+        }
+    };
+}
+
+window.mxModalWidgetStore = ModalWidgetStore.instance;
diff --git a/src/stores/widgets/StopGapWidget.ts b/src/stores/widgets/StopGapWidget.ts
index edce39d0336..0ae835b6031 100644
--- a/src/stores/widgets/StopGapWidget.ts
+++ b/src/stores/widgets/StopGapWidget.ts
@@ -32,6 +32,7 @@ import {
     Widget,
     WidgetApiToWidgetAction,
     WidgetApiFromWidgetAction,
+    IModalWidgetOpenRequest,
 } from "matrix-widget-api";
 import { StopGapWidgetDriver } from "./StopGapWidgetDriver";
 import { EventEmitter } from "events";
@@ -49,6 +50,7 @@ import defaultDispatcher from "../../dispatcher/dispatcher";
 import { ElementWidgetActions } from "./ElementWidgetActions";
 import Modal from "../../Modal";
 import WidgetOpenIDPermissionsDialog from "../../components/views/dialogs/WidgetOpenIDPermissionsDialog";
+import {ModalWidgetStore} from "../ModalWidgetStore";
 
 // TODO: Destroy all of this code
 
@@ -201,7 +203,7 @@ export class StopGapWidget extends EventEmitter {
     }
 
     private onOpenIdReq = async (ev: CustomEvent<IGetOpenIDActionRequest>) => {
-        if (ev?.detail?.widgetId !== this.widgetId) return;
+        ev.preventDefault();
 
         const rawUrl = this.appTileProps.app.url;
         const widgetSecurityKey = WidgetUtils.getWidgetSecurityKey(this.widgetId, rawUrl, this.appTileProps.userWidget);
@@ -249,6 +251,20 @@ export class StopGapWidget extends EventEmitter {
         });
     };
 
+    private onOpenModal = async (ev: CustomEvent<IModalWidgetOpenRequest>) => {
+        ev.preventDefault();
+        if (ModalWidgetStore.instance.canOpenModalWidget()) {
+            ModalWidgetStore.instance.openModalWidget(ev.detail.data, this.mockWidget);
+            this.messaging.transport.reply(ev.detail, {}); // ack
+        } else {
+            this.messaging.transport.reply(ev.detail, {
+                error: {
+                    message: "Unable to open modal at this time",
+                },
+            })
+        }
+    };
+
     public start(iframe: HTMLIFrameElement) {
         if (this.started) return;
         const driver = new StopGapWidgetDriver( this.appTileProps.whitelistCapabilities || []);
@@ -256,6 +272,7 @@ export class StopGapWidget extends EventEmitter {
         this.messaging.on("preparing", () => this.emit("preparing"));
         this.messaging.on("ready", () => this.emit("ready"));
         this.messaging.on(`action:${WidgetApiFromWidgetAction.GetOpenIDCredentials}`, this.onOpenIdReq);
+        this.messaging.on(`action:${WidgetApiFromWidgetAction.OpenModalWidget}`, this.onOpenModal);
         WidgetMessagingStore.instance.storeMessaging(this.mockWidget, this.messaging);
 
         if (!this.appTileProps.userWidget && this.appTileProps.room) {