diff --git a/res/css/structures/_ToastContainer.scss b/res/css/structures/_ToastContainer.scss
index 4c5e746e66..5634a97c53 100644
--- a/res/css/structures/_ToastContainer.scss
+++ b/res/css/structures/_ToastContainer.scss
@@ -51,7 +51,7 @@ limitations under the License.
         &.mx_Toast_hasIcon {
             &::after {
                 content: "";
-                width: 20px;
+                width: 21px;
                 height: 20px;
                 grid-column: 1;
                 grid-row: 1;
@@ -64,6 +64,10 @@ limitations under the License.
                 background-color: $primary-fg-color;
             }
 
+            &.mx_Toast_icon_verification_warning::after {
+                background-image: url("$(res)/img/e2e/warning.svg");
+            }
+
             h2, .mx_Toast_body {
                 grid-column: 2;
             }
diff --git a/src/DeviceListener.js b/src/DeviceListener.js
new file mode 100644
index 0000000000..15ca931fc8
--- /dev/null
+++ b/src/DeviceListener.js
@@ -0,0 +1,92 @@
+/*
+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 { MatrixClientPeg } from './MatrixClientPeg';
+import SettingsStore from './settings/SettingsStore';
+import * as sdk from './index';
+import { _t } from './languageHandler';
+import ToastStore from './stores/ToastStore';
+
+function toastKey(device) {
+    return 'newsession_' + device.deviceId;
+}
+
+export default class DeviceListener {
+    static sharedInstance() {
+        if (!global.mx_DeviceListener) global.mx_DeviceListener = new DeviceListener();
+        return global.mx_DeviceListener;
+    }
+
+    constructor() {
+        // device IDs for which the user has dismissed the verify toast ('Later')
+        this._dismissed = new Set();
+    }
+
+    start() {
+        MatrixClientPeg.get().on('crypto.devicesUpdated', this._onDevicesUpdated);
+        MatrixClientPeg.get().on('deviceVerificationChanged', this._onDeviceVerificationChanged);
+        this.recheck();
+    }
+
+    stop() {
+        if (MatrixClientPeg.get()) {
+            MatrixClientPeg.get().removeListener('crypto.devicesUpdated', this._onDevicesUpdated);
+            MatrixClientPeg.get().removeListener('deviceVerificationChanged', this._onDeviceVerificationChanged);
+        }
+        this._dismissed.clear();
+    }
+
+    dismissVerification(deviceId) {
+        this._dismissed.add(deviceId);
+        this.recheck();
+    }
+
+    _onDevicesUpdated = (users) => {
+        if (!users.includes(MatrixClientPeg.get().getUserId())) return;
+        if (!SettingsStore.isFeatureEnabled("feature_cross_signing")) return;
+        this.recheck();
+    }
+
+    _onDeviceVerificationChanged = (users) => {
+        if (!users.includes(MatrixClientPeg.get().getUserId())) return;
+        if (!SettingsStore.isFeatureEnabled("feature_cross_signing")) return;
+        this.recheck();
+    }
+
+    async recheck() {
+        const cli = MatrixClientPeg.get();
+
+        if (!cli.isCryptoEnabled()) return false;
+
+        const devices = await cli.getStoredDevicesForUser(cli.getUserId());
+        for (const device of devices) {
+            if (device.deviceId == cli.deviceId) continue;
+
+            const deviceTrust = await cli.checkDeviceTrust(cli.getUserId(), device.deviceId);
+            if (deviceTrust.isVerified() || this._dismissed.has(device.deviceId)) {
+                ToastStore.sharedInstance().dismissToast(toastKey(device));
+            } else {
+                ToastStore.sharedInstance().addOrReplaceToast({
+                    key: toastKey(device),
+                    title: _t("New Session"),
+                    icon: "verification_warning",
+                    props: {deviceId: device.deviceId},
+                    component: sdk.getComponent("toasts.NewSessionToast"),
+                });
+            }
+        }
+    }
+}
diff --git a/src/Lifecycle.js b/src/Lifecycle.js
index 0796e326a0..1603c73d25 100644
--- a/src/Lifecycle.js
+++ b/src/Lifecycle.js
@@ -2,6 +2,7 @@
 Copyright 2015, 2016 OpenMarket Ltd
 Copyright 2017 Vector Creations Ltd
 Copyright 2018 New Vector Ltd
+Copyright 2019, 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.
@@ -35,8 +36,10 @@ import { sendLoginRequest } from "./Login";
 import * as StorageManager from './utils/StorageManager';
 import SettingsStore from "./settings/SettingsStore";
 import TypingStore from "./stores/TypingStore";
+import ToastStore from "./stores/ToastStore";
 import {IntegrationManagers} from "./integrations/IntegrationManagers";
 import {Mjolnir} from "./mjolnir/Mjolnir";
+import DeviceListener from "./DeviceListener";
 
 /**
  * Called at startup, to attempt to build a logged-in Matrix session. It tries
@@ -575,6 +578,7 @@ async function startMatrixClient(startSyncing=true) {
     Notifier.start();
     UserActivity.sharedInstance().start();
     TypingStore.sharedInstance().reset(); // just in case
+    ToastStore.sharedInstance().reset();
     if (!SettingsStore.getValue("lowBandwidth")) {
         Presence.start();
     }
@@ -595,6 +599,9 @@ async function startMatrixClient(startSyncing=true) {
         await MatrixClientPeg.assign();
     }
 
+    // This needs to be started after crypto is set up
+    DeviceListener.sharedInstance().start();
+
     // dispatch that we finished starting up to wire up any other bits
     // of the matrix client that cannot be set prior to starting up.
     dis.dispatch({action: 'client_started'});
@@ -651,6 +658,7 @@ export function stopMatrixClient(unsetClient=true) {
     ActiveWidgetStore.stop();
     IntegrationManagers.sharedInstance().stopWatching();
     Mjolnir.sharedInstance().stop();
+    DeviceListener.sharedInstance().stop();
     if (DMRoomMap.shared()) DMRoomMap.shared().stop();
     EventIndexPeg.stop();
     const cli = MatrixClientPeg.get();
diff --git a/src/components/structures/ToastContainer.js b/src/components/structures/ToastContainer.js
index bc74133433..8a05f62e61 100644
--- a/src/components/structures/ToastContainer.js
+++ b/src/components/structures/ToastContainer.js
@@ -22,7 +22,7 @@ import classNames from "classnames";
 export default class ToastContainer extends React.Component {
     constructor() {
         super();
-        this.state = {toasts: []};
+        this.state = {toasts: ToastStore.sharedInstance().getToasts()};
     }
 
     componentDidMount() {
diff --git a/src/components/views/toasts/NewSessionToast.js b/src/components/views/toasts/NewSessionToast.js
new file mode 100644
index 0000000000..f83326121b
--- /dev/null
+++ b/src/components/views/toasts/NewSessionToast.js
@@ -0,0 +1,57 @@
+/*
+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 React from 'react';
+import PropTypes from 'prop-types';
+import * as sdk from "../../../index";
+import { _t } from '../../../languageHandler';
+import Modal from "../../../Modal";
+import { MatrixClientPeg } from '../../../MatrixClientPeg';
+import DeviceListener from '../../../DeviceListener';
+
+export default class VerifySessionToast extends React.PureComponent {
+    static propTypes = {
+        toastKey: PropTypes.string.isRequired,
+        deviceId: PropTypes.string,
+    };
+
+    _onLaterClick = () => {
+        DeviceListener.sharedInstance().dismissVerification(this.props.deviceId);
+    };
+
+    _onVerifyClick = async () => {
+        const cli = MatrixClientPeg.get();
+        const DeviceVerifyDialog = sdk.getComponent('views.dialogs.DeviceVerifyDialog');
+
+        const device = await cli.getStoredDevice(cli.getUserId(), this.props.deviceId);
+
+        Modal.createTrackedDialog('New Session Verify', 'Starting dialog', DeviceVerifyDialog, {
+            userId: MatrixClientPeg.get().getUserId(),
+            device,
+        }, null, /* priority = */ false, /* static = */ true);
+    };
+
+    render() {
+        const FormButton = sdk.getComponent("elements.FormButton");
+        return (<div>
+            <div className="mx_Toast_description">{_t("Other users may not trust it")}</div>
+            <div className="mx_Toast_buttons" aria-live="off">
+                <FormButton label={_t("Later")} kind="danger" onClick={this._onLaterClick} />
+                <FormButton label={_t("Verify")} onClick={this._onVerifyClick} />
+            </div>
+        </div>);
+    }
+}
diff --git a/src/i18n/strings/en_EN.json b/src/i18n/strings/en_EN.json
index f8b17db7c5..4af203177c 100644
--- a/src/i18n/strings/en_EN.json
+++ b/src/i18n/strings/en_EN.json
@@ -85,6 +85,7 @@
     "%(weekDayName)s, %(monthName)s %(day)s %(time)s": "%(weekDayName)s, %(monthName)s %(day)s %(time)s",
     "%(weekDayName)s, %(monthName)s %(day)s %(fullYear)s": "%(weekDayName)s, %(monthName)s %(day)s %(fullYear)s",
     "%(weekDayName)s, %(monthName)s %(day)s %(fullYear)s %(time)s": "%(weekDayName)s, %(monthName)s %(day)s %(fullYear)s %(time)s",
+    "New Session": "New Session",
     "Who would you like to add to this community?": "Who would you like to add to this community?",
     "Warning: any person you add to a community will be publicly visible to anyone who knows the community ID": "Warning: any person you add to a community will be publicly visible to anyone who knows the community ID",
     "Invite new community members": "Invite new community members",
@@ -513,6 +514,9 @@
     "Headphones": "Headphones",
     "Folder": "Folder",
     "Pin": "Pin",
+    "Other users may not trust it": "Other users may not trust it",
+    "Later": "Later",
+    "Verify": "Verify",
     "Decline (%(counter)s)": "Decline (%(counter)s)",
     "Accept <policyLink /> to continue:": "Accept <policyLink /> to continue:",
     "Upload": "Upload",
@@ -1130,7 +1134,6 @@
     "This client does not support end-to-end encryption.": "This client does not support end-to-end encryption.",
     "Messages in this room are not end-to-end encrypted.": "Messages in this room are not end-to-end encrypted.",
     "Messages in this room are end-to-end encrypted.": "Messages in this room are end-to-end encrypted.",
-    "Verify": "Verify",
     "Security": "Security",
     "Sunday": "Sunday",
     "Monday": "Monday",
diff --git a/src/stores/ToastStore.js b/src/stores/ToastStore.js
index f6cc30db67..2c4464813b 100644
--- a/src/stores/ToastStore.js
+++ b/src/stores/ToastStore.js
@@ -31,6 +31,10 @@ export default class ToastStore extends EventEmitter {
         this._toasts = [];
     }
 
+    reset() {
+        this._toasts = [];
+    }
+
     addOrReplaceToast(newToast) {
         const oldIndex = this._toasts.findIndex(t => t.key === newToast.key);
         if (oldIndex === -1) {