diff --git a/.changeset/server-signer-sign.md b/.changeset/server-signer-sign.md
new file mode 100644
index 000000000..6dfc0ee94
--- /dev/null
+++ b/.changeset/server-signer-sign.md
@@ -0,0 +1,5 @@
+---
+"@crossmint/wallets-sdk": minor
+---
+
+`createServerSigner` now returns a `sign(message)` function. It signs a message the API asks the server signer to approve (a transaction or signature approval, or a quorum member's approval message) exactly as the SDK's own server signer does on that chain: raw EIP-191 over 0x-hex on EVM, ed25519 over base58 on Solana, ed25519 over base64 on Stellar. Callers no longer need to rebuild per-chain signing from `keyBytes`.
diff --git a/packages/wallets/src/utils/server-signers/create-server-signer.test.ts b/packages/wallets/src/utils/server-signers/create-server-signer.test.ts
new file mode 100644
index 000000000..90739a751
--- /dev/null
+++ b/packages/wallets/src/utils/server-signers/create-server-signer.test.ts
@@ -0,0 +1,81 @@
+import { describe, it, expect } from "vitest";
+import base58 from "bs58";
+import { ed25519 } from "@noble/curves/ed25519";
+import { recoverMessageAddress } from "viem";
+
+import { encodeStellarPublicKey } from "../stellar";
+import { createServerSigner } from "./createServerSigner";
+
+const params = { secret: "a".repeat(64), projectId: "project-123", environment: "staging" };
+
+describe("createServerSigner().sign", () => {
+    it("EVM: signs the raw hex message so it recovers to the signer's address", async () => {
+        const signer = createServerSigner({ ...params, chain: "base-sepolia" });
+        const message = "0x7b2a0c1d9e";
+
+        const signature = await signer.sign(message);
+
+        expect(signature).toMatch(/^0x[0-9a-f]{130}$/);
+        await expect(
+            recoverMessageAddress({ message: { raw: message }, signature: signature as `0x${string}` })
+        ).resolves.toBe(signer.address);
+    });
+
+    it("EVM: rejects a message that is not 0x-prefixed hex", async () => {
+        const signer = createServerSigner({ ...params, chain: "base-sepolia" });
+
+        await expect(signer.sign("7b2a")).rejects.toThrow("0x-prefixed hex");
+    });
+
+    it("Solana: signs the base58-decoded message with the address's ed25519 key, base58 out", async () => {
+        const signer = createServerSigner({ ...params, chain: "solana" });
+        const messageBytes = new Uint8Array([1, 2, 3, 250, 251, 252]);
+
+        const signature = await signer.sign(base58.encode(messageBytes));
+
+        expect(ed25519.verify(base58.decode(signature), messageBytes, base58.decode(signer.address))).toBe(true);
+    });
+
+    it("Solana: rejects a message that is not base58", async () => {
+        const signer = createServerSigner({ ...params, chain: "solana" });
+
+        await expect(signer.sign("0OIl")).rejects.toThrow("base58");
+    });
+
+    it("Stellar: signs the base64-decoded message with the address's ed25519 key, base64 out", async () => {
+        const signer = createServerSigner({ ...params, chain: "stellar" });
+        // A quorum member's Stellar message is the 32-byte auth-entry preimage hash, base64-encoded.
+        const messageBytes = new Uint8Array(32).map((_, i) => i * 7);
+        const publicKey = ed25519.getPublicKey(signer.keyBytes);
+        expect(encodeStellarPublicKey(publicKey)).toBe(signer.address);
+
+        const signature = await signer.sign(Buffer.from(messageBytes).toString("base64"));
+
+        expect(ed25519.verify(Buffer.from(signature, "base64"), messageBytes, publicKey)).toBe(true);
+    });
+
+    it("Stellar: rejects a message that is not base64", async () => {
+        const signer = createServerSigner({ ...params, chain: "stellar" });
+
+        await expect(signer.sign("not base64!")).rejects.toThrow("base64");
+    });
+
+    it("signs again with the same signer after a first signature", async () => {
+        const signer = createServerSigner({ ...params, chain: "base-sepolia" });
+        const message = "0x01";
+
+        const first = await signer.sign(message);
+
+        await expect(signer.sign(message)).resolves.toBe(first);
+    });
+
+    it("leaves keyBytes intact after signing", async () => {
+        const signer = createServerSigner({ ...params, chain: "solana" });
+        const before = signer.keyBytes.slice();
+
+        await signer.sign(base58.encode(new Uint8Array([9])));
+
+        expect(signer.keyBytes).toEqual(before);
+        expect(before.some((b) => b !== 0)).toBe(true);
+    });
+});
diff --git a/packages/wallets/src/utils/server-signers/createServerSigner.ts b/packages/wallets/src/utils/server-signers/createServerSigner.ts
index da73a65ee..caffefe49 100644
--- a/packages/wallets/src/utils/server-signers/createServerSigner.ts
+++ b/packages/wallets/src/utils/server-signers/createServerSigner.ts
@@ -2,6 +2,7 @@ import type { Chain } from "../../chains/chains";
 import { deriveKeyBytes } from "../server-key-derivation";
 import { deriveServerSignerAddress } from "../../signers/server/helpers/derive-server-signer";
 import { getChainType } from "../../signers/server/helpers/get-chain-type";
+import { assembleServerSigner } from "../../signers/server/assemble-server-signer";
 
 export type CreateServerSignerParams = {
     secret: string;
@@ -14,6 +15,17 @@ export type ServerSigner = {
     type: "server";
     address: string;
     keyBytes: Uint8Array;
+    /**
+     * Signs a message the API asks this signer to approve, such as `approvals.pending[].message` on a transaction
+     * or signature, or a quorum member's `quorumApprovals.pending[].message`. Signs exactly as the SDK does when
+     * this server signer approves on its own, and returns the signature to post under `server:<address>`.
+     *
+     * The message is in the encoding the API returns for the chain:
+     * - EVM: 0x-prefixed hex, signed as a raw EIP-191 personal message; returns a 0x-prefixed signature.
+     * - Solana: base58, signed with ed25519; returns a base58 signature.
+     * - Stellar: base64, signed with ed25519; returns a base64 signature.
+     */
+    sign: (message: string) => Promise<string>;
 };
 
 /**
@@ -25,7 +37,7 @@ export type ServerSigner = {
  * @param params.chain - Target blockchain chain
  * @param params.projectId - Project ID from the API key
  * @param params.environment - Environment from the API key (staging, production)
- * @returns A server signer containing the type, derived address, and key bytes.
+ * @returns A server signer containing the type, derived address, key bytes, and a `sign` function.
  *
  * @example
  * ```ts
@@ -35,7 +47,8 @@ export type ServerSigner = {
  *     projectId: "project-id",
  *     environment: "staging",
  * });
- * // signer = { type: "server", address: "0x...", keyBytes: Uint8Array }
+ * // signer = { type: "server", address: "0x...", keyBytes: Uint8Array, sign }
+ * const signature = await signer.sign(pending.message);
  * ```
  */
 export function createServerSigner(params: CreateServerSignerParams): ServerSigner {
@@ -43,6 +56,18 @@ export function createServerSigner(params: CreateServerSignerParams): ServerSign
     const chainType = getChainType(chain);
     const keyBytes = deriveKeyBytes(secret, projectId, environment, chainType);
     const address = deriveServerSignerAddress(keyBytes, chain);
+    // The adapter wipes the bytes it is given, so hand it a copy and leave `keyBytes` intact for the caller.
+    const adapter = assembleServerSigner(chain, {
+        type: "server",
+        derivedKeyBytes: keyBytes.slice(),
+        locator: `server:${address}`,
+        address,
+    });
 
-    return { type: "server", address, keyBytes };
+    return {
+        type: "server",
+        address,
+        keyBytes,
+        sign: async (message) => (await adapter.signMessage(message)).signature,
+    };
 }