diff --git a/src/MatrixClientPeg.ts b/src/MatrixClientPeg.ts
index ab952c56334..f34753b6798 100644
--- a/src/MatrixClientPeg.ts
+++ b/src/MatrixClientPeg.ts
@@ -340,7 +340,16 @@ class MatrixClientPegClass implements IMatrixClientPeg {
 
         setDeviceIsolationMode(this.matrixClient, SettingsStore.getValue("feature_exclude_insecure_devices"));
 
-        // TODO: device dehydration and whathaveyou
+        // Start dehydration. This code is only for the case where the client
+        // gets restarted, so we only do this if we already have the dehydration
+        // key cached, and we don't have to try to rehydrate a device. If this
+        // is a new login, we will start dehydration after Secret Storage is
+        // unlocked.
+        const crypto = this.matrixClient.getCrypto();
+        if (await crypto?.isDehydrationSupported()) {
+            await crypto.startDehydration({ onlyIfKeyCached: true, rehydrate: false });
+        }
+
         return;
     }
 
diff --git a/test/unit-tests/MatrixClientPeg-test.ts b/test/unit-tests/MatrixClientPeg-test.ts
index c46edad55cf..f87a33e07ee 100644
--- a/test/unit-tests/MatrixClientPeg-test.ts
+++ b/test/unit-tests/MatrixClientPeg-test.ts
@@ -80,10 +80,17 @@ describe("MatrixClientPeg", () => {
 
         it("should initialise the rust crypto library by default", async () => {
             const mockInitRustCrypto = jest.spyOn(testPeg.safeGet(), "initRustCrypto").mockResolvedValue(undefined);
+            const mockStartDehydration = jest.fn();
+            jest.spyOn(testPeg.safeGet(), "getCrypto").mockReturnValue({
+                isDehydrationSupported: jest.fn().mockResolvedValue(true),
+                startDehydration: mockStartDehydration,
+                setDeviceIsolationMode: jest.fn(),
+            });
 
             const cryptoStoreKey = new Uint8Array([1, 2, 3, 4]);
             await testPeg.start({ rustCryptoStoreKey: cryptoStoreKey });
             expect(mockInitRustCrypto).toHaveBeenCalledWith({ storageKey: cryptoStoreKey });
+            expect(mockStartDehydration).toHaveBeenCalledWith({ onlyIfKeyCached: true, rehydrate: false });
         });
 
         it("Should migrate existing login", async () => {
diff --git a/test/unit-tests/components/structures/MatrixChat-test.tsx b/test/unit-tests/components/structures/MatrixChat-test.tsx
index a29834d51f7..66397a7275a 100644
--- a/test/unit-tests/components/structures/MatrixChat-test.tsx
+++ b/test/unit-tests/components/structures/MatrixChat-test.tsx
@@ -129,6 +129,7 @@ describe("<MatrixChat />", () => {
         getCrypto: jest.fn().mockReturnValue({
             getVerificationRequestsToDeviceInProgress: jest.fn().mockReturnValue([]),
             isCrossSigningReady: jest.fn().mockReturnValue(false),
+            isDehydrationSupported: jest.fn().mockReturnValue(false),
             getUserDeviceInfo: jest.fn().mockReturnValue(new Map()),
             getUserVerificationStatus: jest.fn().mockResolvedValue(new UserVerificationStatus(false, false, false)),
             getVersion: jest.fn().mockReturnValue("1"),
@@ -1006,6 +1007,7 @@ describe("<MatrixChat />", () => {
                     resetKeyBackup: jest.fn(),
                     isEncryptionEnabledInRoom: jest.fn().mockResolvedValue(false),
                     checkKeyBackupAndEnable: jest.fn().mockResolvedValue(null),
+                    isDehydrationSupported: jest.fn().mockReturnValue(false),
                 };
                 loginClient.getCrypto.mockReturnValue(mockCrypto as any);
             });