diff --git a/scripts/copy-res.js b/scripts/copy-res.js
index dc55cb1a..ff8ee458 100755
--- a/scripts/copy-res.js
+++ b/scripts/copy-res.js
@@ -61,6 +61,7 @@ const COPY_LIST = [
     // in the bundle dir with the js to avoid caching issues giving us wasm that
     // doesn't match our js, but I cannot find any way to get webpack to do this.
     ["node_modules/olm/olm.wasm", "webapp", { directwatch: 1 }],
+    ["node_modules/olm/olm_legacy.js", "webapp", { directwatch: 1 }],
     ["./config.json", "webapp", { directwatch: 1 }],
 ];
 
diff --git a/src/vector/index.js b/src/vector/index.js
index 8fde38f5..57d1f87a 100644
--- a/src/vector/index.js
+++ b/src/vector/index.js
@@ -229,19 +229,7 @@ async function loadApp() {
 
     window.addEventListener('hashchange', onHashChange);
 
-    /* Start loading Olm. Note that we *don't* wait for this to load: the
-     * js-sdk will also call this and actually wait on the promise before it
-     * tries to use the library. It can be loading its wasm while the rest of
-     * the app loads though.
-     *
-     * We also need to tell the Olm js to look for its wasm file at the same
-     * level as index.html. It really should be in the same place as the js,
-     * ie. in the bundle directory, to avoid caching issues, but as far as I
-     * can tell this is completely impossible with webpack.
-     */
-    Olm.init({
-        locateFile: () => 'olm.wasm',
-    });
+    await loadOlm();
 
     await loadLanguage();
 
@@ -365,6 +353,42 @@ async function loadApp() {
     }
 }
 
+function loadOlm() {
+    /* Load Olm. We try the WebAssembly version first, and then the legacy,
+     * asm.js version if that fails. For this reason we need to wait for this
+     * to finish before continuing to load the rest of the app. In future
+     * we could somehow pass a promise down to react-sdk and have it wait on
+     * that so olm can be loading in parallel with the rest of the app.
+     *
+     * We also need to tell the Olm js to look for its wasm file at the same
+     * level as index.html. It really should be in the same place as the js,
+     * ie. in the bundle directory, to avoid caching issues, but as far as I
+     * can tell this is completely impossible with webpack.
+     */
+    return Olm.init({
+        locateFile: () => 'olm.wasm',
+    }).then(() => {
+        console.log("Using WebAssembly Olm");
+    }).catch((e) => {
+        console.log("Failed to load Olm: trying legacy version");
+        return new Promise((resolve, reject) => {
+            const s = document.createElement('script');
+            s.src = 'olm_legacy.js';
+            s.onload = resolve;
+            s.onerror = reject;
+            document.body.appendChild(s);
+        }).then(() => {
+            // Init window.Olm, ie. the one just loaded by the script tag,
+            // not 'Olm' which is still the failed wasm version.
+            return window.Olm.init();
+        }).then(() => {
+            console.log("Using legacy Olm");
+        }).catch((e) => {
+            console.log("Both WebAssembly and asm.js Olm failed!", e);
+        });
+    });
+}
+
 async function loadLanguage() {
     const prefLang = SettingsStore.getValue("language", null, /*excludeDefault=*/true);
     let langs = [];