diff --git a/src/observable/index.js b/src/observable/index.js
index eb6f1579..351c25b8 100644
--- a/src/observable/index.js
+++ b/src/observable/index.js
@@ -17,6 +17,7 @@ limitations under the License.
 import {SortedMapList} from "./list/SortedMapList.js";
 import {FilteredMap} from "./map/FilteredMap.js";
 import {MappedMap} from "./map/MappedMap.js";
+import {JoinedMap} from "./map/JoinedMap.js";
 import {BaseObservableMap} from "./map/BaseObservableMap.js";
 // re-export "root" (of chain) collections
 export { ObservableArray } from "./list/ObservableArray.js";
@@ -38,5 +39,9 @@ Object.assign(BaseObservableMap.prototype, {
 
     filterValues(filter) {
         return new FilteredMap(this, filter);
+    },
+
+    join(...otherMaps) {
+        return new JoinedMap([this].concat(otherMaps));
     }
 });
diff --git a/src/observable/map/JoinedMap.js b/src/observable/map/JoinedMap.js
new file mode 100644
index 00000000..b50a524a
--- /dev/null
+++ b/src/observable/map/JoinedMap.js
@@ -0,0 +1,112 @@
+/*
+Copyright 2021 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 {BaseObservableMap} from "./BaseObservableMap.js";
+
+export class JoinedMap extends BaseObservableMap {
+    constructor(sources) {
+        super();
+        this._sources = sources;
+    }
+
+    onAdd(key, value) {
+        this.emitAdd(key, value);
+    }
+
+    onRemove(key, value) {
+        this.emitRemove(key, value);
+    }
+
+    onUpdate(key, value, params) {
+        this.emitUpdate(key, value, params);
+    }
+
+    onSubscribeFirst() {
+        this._subscriptions = this._sources.map(source => source.subscribe(this));
+        super.onSubscribeFirst();
+    }
+
+    onUnsubscribeLast() {
+        super.onUnsubscribeLast();
+        for (const s of this._subscriptions) {
+            s();
+        }
+    }
+
+    onReset() {
+        this.emitReset();
+    }
+
+    [Symbol.iterator]() {
+        return new JoinedIterator(this._sources);
+    }
+
+    get size() {
+        return this._sources.reduce((sum, s) => sum + s.size, 0);
+    }
+
+    get(key) {
+        for (const s of this._sources) {
+            const value = s.get(key);
+            if (value) {
+                return value;
+            }
+        }
+        return null;
+    }
+}
+
+class JoinedIterator {
+    constructor(sources) {
+        this._sources = sources;
+        this._sourceIndex = -1;
+        this._currentIterator = null;
+    }
+
+    next() {
+        let result;
+        while (!result) {
+            if (!this._currentIterator) {
+                this._sourceIndex += 1;
+                if (this._sources.length <= this._sourceIndex) {
+                    return {done: true};
+                }
+                this._currentIterator = this._sources[this._sourceIndex][Symbol.iterator]();
+            }
+            const sourceResult = this._currentIterator.next();
+            if (sourceResult.done) {
+                this._currentIterator = null;
+                continue;
+            } else {
+                result = sourceResult;
+            }
+        }
+        return result;
+    }
+}
+
+export function tests() {
+    return {
+        "joined iterator": assert => {
+            const it = new JoinedIterator([[1, 2], [3, 4]]);
+            assert.equal(it.next().value, 1);
+            assert.equal(it.next().value, 2);
+            assert.equal(it.next().value, 3);
+            assert.equal(it.next().value, 4);
+            assert.equal(it.next().done, true);
+        }
+    };
+}