diff --git a/NEWS.md b/NEWS.md
index ed9baf8361..7c5e241cd2 100644
--- a/NEWS.md
+++ b/NEWS.md
@@ -19,10 +19,12 @@
 - URLs containing special characters like commas in query string are now handled correctly ([#1082](https://github.com/SSilence/selfoss/pull/1082))
 - Set 60 second timeout to spout HTTP requests to prevent a single feed blocking other updates ([#1104](https://github.com/SSilence/selfoss/issues/1104))
 - Significantly improved accessibility ([#1133](https://github.com/SSilence/selfoss/pull/1133), [#1134](https://github.com/SSilence/selfoss/pull/1134) and [#1141](https://github.com/SSilence/selfoss/pull/1141))
+- Fixed marking more than 1000 items as read at the same time ([#1182](https://github.com/SSilence/selfoss/issues/1182))
 
 ### API changes
 - `tags` attribute is now consistently array of strings, numbers are numbers and booleans are booleans. **This might break third-party clients that have not updated yet.** ([#948](https://github.com/SSilence/selfoss/pull/948))
 - API is now versioned separately from selfoss and follows [semantic versioning](https://semver.org/) ([#1137](https://github.com/SSilence/selfoss/pull/1137))
+- *API 2.21.0*: `/mark` now accepts list of item IDs encoded as JSON. Requests using `application/x-www-form-urlencoded` are deprecated. ([#1182](https://github.com/SSilence/selfoss/pull/1182))
 
 ### Customization changes
 - `selfoss.shares.register` changed its signature: it no longer takes a boolean argument, and the callback is expected to open a window itself, instead of returning a URL. The `register` method now also expects a label and a HTML code of an icon (you can use a `<img>` tag, inline `<svg>`, emoji, etc.).
diff --git a/assets/js/selfoss-base.js b/assets/js/selfoss-base.js
index 499e4d491b..7b4ff2d944 100644
--- a/assets/js/selfoss-base.js
+++ b/assets/js/selfoss-base.js
@@ -603,9 +603,8 @@ var selfoss = {
             url: 'mark',
             type: 'POST',
             dataType: 'json',
-            data: {
-                ids: ids
-            },
+            contentType: 'application/json; charset=utf-8',
+            data: JSON.stringify(ids),
             success: function() {
                 selfoss.db.setOnline();
                 displayNextUnread();
diff --git a/docs/api-description.json b/docs/api-description.json
index 7c9fcc8f87..f81f38aa77 100644
--- a/docs/api-description.json
+++ b/docs/api-description.json
@@ -3,7 +3,7 @@
   "servers": [],
   "info": {
     "description": "You can access selfoss by using the same backend as selfoss user interface: The RESTful HTTP JSON API. There are a few urls where you can get information from selfoss and some for updating data. Assume you want all tags for rendering this in your own app. You have to make an HTTP GET call on the url /tags:\n\n```\nGET http://yourselfossurl.com/tags\n```\nThe result is following JSON formatted response (in this example two tags “blog” and “deviantart” are available:\n\n```\n[{\"tag\":\"blog\",\"color\":\"#251f10\",\"unread\":\"1\"},\n{\"tag\":\"deviantart\",\"color\":\"#e78e5c\",\"unread\":\"0\"}]\n```\n\nFollowing docs shows you which calls are possible and which response you can expect.",
-    "version": "2.20.0",
+    "version": "2.21.0",
     "title": "selfoss"
   },
   "tags": [
@@ -330,8 +330,18 @@
         },
         "requestBody": {
           "content": {
+            "application/json": {
+              "schema": {
+                "description": "a list of all ids for marking as read",
+                "type": "array",
+                "items": {
+                  "type": "integer"
+                }
+              }
+            },
             "application/x-www-form-urlencoded": {
               "schema": {
+                "deprecated": true,
                 "type": "object",
                 "properties": {
                   "ids": {
diff --git a/src/controllers/Items.php b/src/controllers/Items.php
index 375080e5dc..62fb630a9c 100644
--- a/src/controllers/Items.php
+++ b/src/controllers/Items.php
@@ -43,8 +43,14 @@ public function mark(Base $f3, array $params) {
 
         if (isset($params['item'])) {
             $lastid = $params['item'];
-        } elseif (isset($_POST['ids'])) {
-            $lastid = $_POST['ids'];
+        } else {
+            $headers = \F3::get('HEADERS');
+            if (isset($headers['Content-Type']) && strpos($headers['Content-Type'], 'application/json') === 0) {
+                $body = \F3::get('BODY');
+                $lastid = json_decode($body, true);
+            } elseif (isset($_POST['ids'])) {
+                $lastid = $_POST['ids'];
+            }
         }
 
         // validate id or ids