diff --git a/lib/types/identity-search/auth-message-types.js b/lib/types/identity-search/auth-message-types.js
new file mode 100644
--- /dev/null
+++ b/lib/types/identity-search/auth-message-types.js
@@ -0,0 +1,21 @@
+// @flow
+
+import type { TInterface } from 'tcomb';
+import t from 'tcomb';
+
+import { tShape, tString } from '../../utils/validation-utils.js';
+
+export type AuthMessage = {
+  +type: 'AuthMessage',
+  +userID: string,
+  +deviceID: string,
+  +accessToken: string,
+};
+
+export const authMessageValidator: TInterface<AuthMessage> =
+  tShape<AuthMessage>({
+    type: tString('AuthMessage'),
+    userID: t.String,
+    deviceID: t.String,
+    accessToken: t.String,
+  });
diff --git a/lib/types/identity-search/messages.js b/lib/types/identity-search/messages.js
new file mode 100644
--- /dev/null
+++ b/lib/types/identity-search/messages.js
@@ -0,0 +1,61 @@
+// @flow
+
+import type { TUnion } from 'tcomb';
+import t from 'tcomb';
+
+import {
+  type AuthMessage,
+  authMessageValidator,
+} from './auth-message-types.js';
+import {
+  type SearchQuery,
+  searchQueryValidator,
+} from './search-query-types.js';
+import {
+  type SearchResult,
+  searchResultValidator,
+} from './search-result-types.js';
+import {
+  type ConnectionInitializationResponse,
+  connectionInitializationResponseValidator,
+} from '../websocket/connection-initialization-response-types.js';
+import {
+  type Heartbeat,
+  heartbeatValidator,
+} from '../websocket/heartbeat-types.js';
+
+export const identitySearchMessageTypes = Object.freeze({
+  AUTH_MESSAGE: 'AuthMessage',
+  SEARCH_QUERY: 'SearchQuery',
+  HEARTBEAT: 'Heartbeat',
+  CONNECTION_INITIALIZATION_RESPONSE: 'ConnectionInitializationResponse',
+  SEARCH_RESULT: 'SearchResult',
+});
+
+/*
+ * This file defines types and validation for messages exchanged
+ * with the Identity Search WebSocket server.
+ * The definitions in this file should remain in sync
+ * with the structures defined in the corresponding
+ * Rust file at `shared/identity_search_messages/src/messages/mod.rs`.
+ *
+ * If you edit the definitions in one file,
+ * please make sure to update the corresponding definitions in the other.
+ *
+ */
+
+export const identitySearchMessageValidator: TUnion<IdentitySearchMessage> =
+  t.union([
+    authMessageValidator,
+    searchQueryValidator,
+    heartbeatValidator,
+    connectionInitializationResponseValidator,
+    searchResultValidator,
+  ]);
+
+export type IdentitySearchMessage =
+  | AuthMessage
+  | SearchQuery
+  | Heartbeat
+  | ConnectionInitializationResponse
+  | SearchResult;
diff --git a/lib/types/identity-search/search-query-types.js b/lib/types/identity-search/search-query-types.js
new file mode 100644
--- /dev/null
+++ b/lib/types/identity-search/search-query-types.js
@@ -0,0 +1,20 @@
+// @flow
+
+import type { TInterface } from 'tcomb';
+import t from 'tcomb';
+
+import { tShape, tString } from '../../utils/validation-utils.js';
+
+export type SearchQuery = Prefix;
+
+export type Prefix = {
+  +type: 'Prefix',
+  +prefix: string,
+};
+
+export const prefixValidator: TInterface<Prefix> = tShape<Prefix>({
+  type: tString('Prefix'),
+  prefix: t.String,
+});
+
+export const searchQueryValidator: TInterface<Prefix> = prefixValidator;
diff --git a/lib/types/identity-search/search-result-types.js b/lib/types/identity-search/search-result-types.js
new file mode 100644
--- /dev/null
+++ b/lib/types/identity-search/search-result-types.js
@@ -0,0 +1,27 @@
+// @flow
+
+import type { TInterface } from 'tcomb';
+import t from 'tcomb';
+
+import { tShape, tString } from '../../utils/validation-utils.js';
+
+export type User = {
+  +userID: string,
+  +username: string,
+};
+
+export type SearchResult = {
+  +type: 'SearchResult',
+  +payload: $ReadOnlyArray<User>,
+};
+
+export const userValidator: TInterface<User> = tShape<User>({
+  userID: t.String,
+  username: t.String,
+});
+
+export const searchResultValidator: TInterface<SearchResult> =
+  tShape<SearchResult>({
+    type: tString('SearchResult'),
+    payload: t.list(userValidator),
+  });