diff --git a/spec/integ/crypto/crypto.spec.ts b/spec/integ/crypto/crypto.spec.ts
index 96d809aea9..386f3dd531 100644
--- a/spec/integ/crypto/crypto.spec.ts
+++ b/spec/integ/crypto/crypto.spec.ts
@@ -3121,6 +3121,32 @@ describe.each(Object.entries(CRYPTO_BACKENDS))("crypto (%s)", (backend: string,
                 const mskId = await aliceClient.getCrypto()!.getCrossSigningKeyId(CrossSigningKey.Master)!;
                 expect(signatures![aliceClient.getUserId()!][`ed25519:${mskId}`]).toBeDefined();
             });
+
+            newBackendOnly("should upload existing megolm backup key to a new 4S store", async () => {
+                const backupKeyTo4SPromise = awaitMegolmBackupKeyUpload();
+
+                // we need these to set up the mocks but we don't actually care whether they
+                // resolve because we're not testing those things in this test.
+                awaitCrossSigningKeyUpload("master");
+                awaitCrossSigningKeyUpload("user_signing");
+                awaitCrossSigningKeyUpload("self_signing");
+                awaitSecretStorageKeyStoredInAccountData();
+
+                mockSetupCrossSigningRequests();
+                mockSetupMegolmBackupRequests("1");
+
+                await aliceClient.getCrypto()!.bootstrapCrossSigning({});
+                await aliceClient.getCrypto()!.resetKeyBackup();
+
+                await aliceClient.getCrypto()!.bootstrapSecretStorage({
+                    setupNewSecretStorage: true,
+                    createSecretStorageKey,
+                    setupNewKeyBackup: false,
+                });
+
+                await backupKeyTo4SPromise;
+                expect(accountDataAccumulator.accountDataEvents.get("m.megolm_backup.v1")).toBeDefined();
+            });
         });
 
         describe("Manage Key Backup", () => {
diff --git a/spec/unit/rust-crypto/rust-crypto.spec.ts b/spec/unit/rust-crypto/rust-crypto.spec.ts
index fc9b571acb..6510bcc854 100644
--- a/spec/unit/rust-crypto/rust-crypto.spec.ts
+++ b/spec/unit/rust-crypto/rust-crypto.spec.ts
@@ -705,6 +705,119 @@ describe("RustCrypto", () => {
         expect(resetKeyBackup.mock.calls).toHaveLength(2);
     });
 
+    describe("upload existing key backup key to new 4S store", () => {
+        const secretStorageCallbacks = {
+            getSecretStorageKey: async (keys: any, name: string) => {
+                return [[...Object.keys(keys.keys)][0], new Uint8Array(32)];
+            },
+        } as SecretStorageCallbacks;
+        let secretStorage: ServerSideSecretStorageImpl;
+
+        let backupAuthData: any;
+        let backupAlg: string;
+
+        const fetchMock = {
+            authedRequest: jest.fn().mockImplementation((method, path, query, body) => {
+                if (path === "/room_keys/version") {
+                    if (method === "POST") {
+                        backupAuthData = body["auth_data"];
+                        backupAlg = body["algorithm"];
+                        return Promise.resolve({ version: "1", algorithm: backupAlg, auth_data: backupAuthData });
+                    } else if (method === "GET" && backupAuthData) {
+                        return Promise.resolve({ version: "1", algorithm: backupAlg, auth_data: backupAuthData });
+                    }
+                }
+                return Promise.resolve({});
+            }),
+        };
+
+        beforeEach(() => {
+            backupAuthData = undefined;
+            backupAlg = "";
+
+            secretStorage = new ServerSideSecretStorageImpl(new DummyAccountDataClient(), secretStorageCallbacks);
+        });
+
+        it("bootstrapSecretStorage saves megolm backup key if already cached", async () => {
+            const rustCrypto = await makeTestRustCrypto(
+                fetchMock as unknown as MatrixHttpApi<any>,
+                testData.TEST_USER_ID,
+                undefined,
+                secretStorage,
+            );
+
+            async function createSecretStorageKey() {
+                return {
+                    keyInfo: {} as AddSecretStorageKeyOpts,
+                    privateKey: new Uint8Array(32),
+                };
+            }
+
+            await rustCrypto.resetKeyBackup();
+
+            const storeSpy = jest.spyOn(secretStorage, "store");
+
+            await rustCrypto.bootstrapSecretStorage({
+                createSecretStorageKey,
+                setupNewSecretStorage: true,
+                setupNewKeyBackup: false,
+            });
+
+            expect(storeSpy).toHaveBeenCalledWith("m.megolm_backup.v1", expect.anything());
+        });
+
+        it("bootstrapSecretStorage doesn't try to save megolm backup key not in cache", async () => {
+            const mockOlmMachine = {
+                isBackupEnabled: jest.fn().mockResolvedValue(false),
+                sign: jest.fn().mockResolvedValue({
+                    asJSON: jest.fn().mockReturnValue("{}"),
+                }),
+                saveBackupDecryptionKey: jest.fn(),
+                crossSigningStatus: jest.fn().mockResolvedValue({
+                    hasMaster: true,
+                    hasSelfSigning: true,
+                    hasUserSigning: true,
+                }),
+                exportCrossSigningKeys: jest.fn().mockResolvedValue({
+                    masterKey: "sosecret",
+                    userSigningKey: "secrets",
+                    self_signing_key: "ssshhh",
+                }),
+                getBackupKeys: jest.fn().mockResolvedValue({}),
+                verifyBackup: jest.fn().mockResolvedValue({ trusted: jest.fn().mockReturnValue(false) }),
+            } as unknown as OlmMachine;
+
+            const rustCrypto = new RustCrypto(
+                logger,
+                mockOlmMachine,
+                fetchMock as unknown as MatrixHttpApi<any>,
+                TEST_USER,
+                TEST_DEVICE_ID,
+                secretStorage,
+                {} as CryptoCallbacks,
+            );
+
+            async function createSecretStorageKey() {
+                return {
+                    keyInfo: {} as AddSecretStorageKeyOpts,
+                    privateKey: new Uint8Array(32),
+                };
+            }
+
+            await rustCrypto.resetKeyBackup();
+
+            const storeSpy = jest.spyOn(secretStorage, "store");
+
+            await rustCrypto.bootstrapSecretStorage({
+                createSecretStorageKey,
+                setupNewSecretStorage: true,
+                setupNewKeyBackup: false,
+            });
+
+            expect(storeSpy).not.toHaveBeenCalledWith("m.megolm_backup.v1", expect.anything());
+        });
+    });
+
     it("isSecretStorageReady", async () => {
         const mockSecretStorage = {
             getDefaultKeyId: jest.fn().mockResolvedValue(null),
diff --git a/src/rust-crypto/rust-crypto.ts b/src/rust-crypto/rust-crypto.ts
index 86853a9867..79308e8369 100644
--- a/src/rust-crypto/rust-crypto.ts
+++ b/src/rust-crypto/rust-crypto.ts
@@ -841,11 +841,46 @@ export class RustCrypto extends TypedEventEmitter<RustCryptoEvents, CryptoEventH
             await this.secretStorage.store("m.cross_signing.self_signing", crossSigningPrivateKeys.self_signing_key);
         }
 
-        if (setupNewKeyBackup) {
+        // likewise with the key backup key: if we have one, store it in secret storage (if it's not already there)
+        // also don't bother storing it if we're about to set up a new backup
+        if (!setupNewKeyBackup) {
+            await this.saveBackupKeyToStorage();
+        } else {
             await this.resetKeyBackup();
         }
     }
 
+    /**
+     * If we have a backup key for the current, trusted backup in cache,
+     * and we have secret storage active, save it to secret storage.
+     */
+    private async saveBackupKeyToStorage(): Promise<void> {
+        const keyBackupInfo = await this.backupManager.getServerBackupInfo();
+        if (!keyBackupInfo || !keyBackupInfo.version) {
+            logger.info("Not saving backup key to secret storage: no backup info");
+            return;
+        }
+
+        const backupKeys: RustSdkCryptoJs.BackupKeys = await this.olmMachine.getBackupKeys();
+        if (!backupKeys.decryptionKey) {
+            logger.info("Not saving backup key to secret storage: no backup key");
+            return;
+        }
+
+        if (!decryptionKeyMatchesKeyBackupInfo(backupKeys.decryptionKey, keyBackupInfo)) {
+            logger.info("Not saving backup key to secret storage: decryption key does not match backup info");
+        }
+
+        const backupKeyFromStorage = await this.secretStorage.get("m.megolm_backup.v1");
+        const backupKeyBase64 = backupKeys.decryptionKey.toBase64();
+
+        // The backup version that the key corresponds to isn't saved in 4S so if it's different, we must assume
+        // it's stale and overwrite.
+        if (backupKeyFromStorage !== backupKeyBase64) {
+            await this.secretStorage.store("m.megolm_backup.v1", backupKeyBase64);
+        }
+    }
+
     /**
      * Add the secretStorage key to the secret storage
      * - The secret storage key must have the `keyInfo` field filled