diff --git a/build-settings/check-type-definitions.ts b/build-settings/check-type-definitions.ts
new file mode 100755
index 00000000..460e4df6
--- /dev/null
+++ b/build-settings/check-type-definitions.ts
@@ -0,0 +1,59 @@
+#!/usr/bin/env -S node -r @swc-node/register
+/* eslint-disable no-console */
+/**
+ * Check type definition files for anything that doesn't look right.
+ *
+ * If our packages don't export all the types that other packages reference,
+ * even indirectly, then their type definitions will import them from the source
+ * files instead. Since we don't want to ship source code in every package,
+ * we want to guard against this.
+ *
+ * This script should be run after `yarn build:types`. It will scan the type
+ * definitions of each package for any types that are being incorrectly
+ * imported from other the source code of other packages, and flag them,
+ * exiting with a non-zero status code if any are found.
+ */
+import * as fs from "fs";
+import * as path from "path";
+import * as fglob from "fast-glob";
+
+const rootDir = path.join(__dirname, "..");
+const packagesDir = path.join(rootDir, "packages");
+
+// Find all the type definition files in the packages dist directories.
+const typeDefinitionFiles = fglob.sync("**/*.d.ts", {
+    cwd: packagesDir,
+    onlyFiles: true,
+});
+
+let foundErrors = false;
+// Scan each one for any imports of types from source.
+for (const typeDefinitionFile of typeDefinitionFiles) {
+    const regexpImportSrc =
+        /import\(".+\/(wonder-stuff-.+)\/src\/.+"\)\.([a-zA-Z]+)/g;
+    const content = fs.readFileSync(
+        path.join(packagesDir, typeDefinitionFile),
+        "utf-8",
+    );
+    const lines = content.split("\n");
+    let match;
+    for (let line = 0; line < lines.length; line++) {
+        while ((match = regexpImportSrc.exec(lines[line]))) {
+            foundErrors = true;
+            const position = match.index;
+            const lineNo = line + 1;
+            const refPath = path.join("packages", typeDefinitionFile);
+            console.error(`${refPath}:${lineNo}:${position}`);
+            console.error(
+                `    Incorrectly imported type ${match[2]} from ${match[1]} source`,
+            );
+            console.error(
+                `    Update the package ${match[1]} to export the type ${match[2]}\n`,
+            );
+        }
+    }
+}
+
+if (foundErrors) {
+    process.exit(1);
+}
diff --git a/package.json b/package.json
index 6eeee9e3..2e3bb875 100644
--- a/package.json
+++ b/package.json
@@ -83,7 +83,7 @@
     "rollup": "rollup -c build-settings/rollup.config.mjs",
     "build": "yarn rollup",
     "build:prodsizecheck": "yarn rollup --configPlatforms='browser' --configFormats='esm' --configEnvironment='production'",
-    "build:types": "yarn tsc --build --verbose tsconfig-build.json && ./build-settings/remove-files-not-to-be-packaged.ts",
+    "build:types": "yarn tsc --build --verbose tsconfig-build.json && ./build-settings/remove-files-not-to-be-packaged.ts && ./build-settings/check-type-definitions.ts",
     "build:docs": "typedoc",
     "watch": "yarn rollup --watch",
     "clean": "rm -rf packages/wonder-stuff-*/dist && rm -rf packages/wonder-stuff-*/node_modules && rm -f packages/*/*.tsbuildinfo",