diff --git a/packages/sdk/README.md b/packages/sdk/README.md
index 09b4064..e25dee5 100644
--- a/packages/sdk/README.md
+++ b/packages/sdk/README.md
@@ -182,6 +182,37 @@ await client.executeSwapQuote({
 });
 ```
 
+## Authentication
+
+Some Across API features require authentication via an API key. You can provide the
+key at the client level or per-request.
+
+### Client-level API key
+
+```ts
+const client = createAcrossClient({
+  integratorId: "0xdead",
+  chains: [mainnet, optimism, arbitrum],
+  apiKey: "your-api-key",
+});
+
+// All authenticated requests will use this key automatically
+const quote = await client.getSwapQuote({ route, amount });
+```
+
+### Per-request API key
+
+```ts
+// Override or provide the key for a single request
+const quote = await client.getSwapQuote({
+  route,
+  amount,
+  apiKey: "per-request-key",
+});
+```
+
+The key is sent as a `Bearer` token in the `Authorization` header.
+
 ## Deposit details
 
 TODO
diff --git a/packages/sdk/src/actions/getSwapQuote.ts b/packages/sdk/src/actions/getSwapQuote.ts
index c794fdd..1701f52 100644
--- a/packages/sdk/src/actions/getSwapQuote.ts
+++ b/packages/sdk/src/actions/getSwapQuote.ts
@@ -46,6 +46,11 @@ export type GetSwapQuoteParams = Omit<
    * [Optional] The Across API URL to use. Defaults to the mainnet API URL.
    */
   apiUrl?: string;
+  /**
+   * [Optional] An API key for accessing authenticated features. Will be sent
+   * as a Bearer token in the Authorization header.
+   */
+  apiKey?: string;
 };
 
 /**
@@ -57,7 +62,7 @@ export type GetSwapQuoteParams = Omit<
 export async function getSwapQuote(
   params: GetSwapQuoteParams,
 ): Promise<SwapApprovalApiResponse> {
-  const { logger, apiUrl = MAINNET_API_URL, ...otherParams } = params;
+  const { logger, apiUrl = MAINNET_API_URL, apiKey, ...otherParams } = params;
 
   logger?.debug("Getting swap quote with params:", otherParams);
 
@@ -83,12 +88,15 @@ export async function getSwapQuote(
           value: action.value?.toString() ?? "0",
         })),
       },
+      apiKey,
     );
   } else {
     data = await fetchAcrossApi<SwapApprovalApiResponse>(
       url,
       queryParams,
       logger,
+      undefined,
+      apiKey,
     );
   }
 
diff --git a/packages/sdk/src/client.ts b/packages/sdk/src/client.ts
index 12ce434..1257799 100644
--- a/packages/sdk/src/client.ts
+++ b/packages/sdk/src/client.ts
@@ -123,6 +123,11 @@ export type AcrossClientOptions = {
    * Defaults to `3_000` milliseconds.
    */
   pollingInterval?: number;
+  /**
+   * An API key for accessing authenticated Across API features. Will be sent as
+   * a Bearer token in the Authorization header.
+   */
+  apiKey?: string;
   /**
    * Tenderly related options. Can be used for additional debugging support on Tenderly.
    * @see https://tenderly.co/transaction-simulator
@@ -164,6 +169,7 @@ export class AcrossClient {
   private walletClient?: ConfiguredWalletClient;
   private apiUrl: string;
   private indexerUrl: string;
+  private apiKey?: string;
 
   logger: LoggerT;
 
@@ -197,6 +203,7 @@ export class AcrossClient {
     this.indexerUrl =
       args?.useTestnet === true ? TESTNET_INDEXER_API : MAINNET_INDEXER_API;
     this.apiUrl = args?.useTestnet === true ? TESTNET_API_URL : MAINNET_API_URL;
+    this.apiKey = args.apiKey;
     this.logger =
       args?.logger ??
       new DefaultLogger(args?.logLevel ?? CLIENT_DEFAULTS.logLevel);
@@ -655,7 +662,7 @@ export class AcrossClient {
    * @returns See {@link SwapApprovalApiResponse}.
    */
   async getSwapQuote(
-    params: MakeOptional<GetSwapQuoteParams, "logger" | "apiUrl">,
+    params: MakeOptional<GetSwapQuoteParams, "logger" | "apiUrl" | "apiKey">,
   ): Promise<SwapApprovalApiResponse> {
     try {
       const quote = await getSwapQuote({
@@ -664,6 +671,7 @@ export class AcrossClient {
         integratorId: params?.integratorId ?? this.integratorId,
         logger: params?.logger ?? this.logger,
         apiUrl: params?.apiUrl ?? this.apiUrl,
+        apiKey: params?.apiKey ?? this.apiKey,
       });
       return quote;
     } catch (e) {
diff --git a/packages/sdk/src/utils/fetch.ts b/packages/sdk/src/utils/fetch.ts
index 125cad0..93acc27 100644
--- a/packages/sdk/src/utils/fetch.ts
+++ b/packages/sdk/src/utils/fetch.ts
@@ -64,12 +64,21 @@ function makeFetcher(
     params: Record<string, ParamBaseValue | Array<ParamBaseValue>>,
     logger?: LoggerT,
     body?: Record<string, unknown>,
+    apiKey?: string,
   ): Promise<ResBody> => {
     const searchParams = buildSearchParams(params);
     const url = `${apiUrl}?${searchParams}`;
 
     logger?.debug(`Fetching ${name}...`, url);
 
+    const headers: Record<string, string> = {};
+    if (method === "POST") {
+      headers["Content-Type"] = "application/json";
+    }
+    if (apiKey) {
+      headers["Authorization"] = `Bearer ${apiKey}`;
+    }
+
     const res = await fetch(url, {
       method,
       body:
@@ -78,12 +87,7 @@ function makeFetcher(
               typeof value === "bigint" ? value.toString() : value,
             )
           : undefined,
-      headers:
-        method === "POST"
-          ? {
-              "Content-Type": "application/json",
-            }
-          : undefined,
+      headers: Object.keys(headers).length > 0 ? headers : undefined,
     });
 
     // Try to parse the response as JSON. If it fails, parse it as text.
diff --git a/packages/sdk/test/common/sdk.ts b/packages/sdk/test/common/sdk.ts
index d66ee74..21abac6 100644
--- a/packages/sdk/test/common/sdk.ts
+++ b/packages/sdk/test/common/sdk.ts
@@ -8,7 +8,6 @@ import {
   linea,
   lisk,
   scroll,
-  redstone,
   zora,
   sepolia,
 } from "viem/chains";
@@ -24,7 +23,6 @@ export const MAINNET_SUPPORTED_CHAINS = [
   linea,
   lisk,
   scroll,
-  redstone,
   zora,
 ] as const;