diff --git a/.changeset/funny-cars-dance.md b/.changeset/funny-cars-dance.md
new file mode 100644
index 00000000000..ae1cad3c6e9
--- /dev/null
+++ b/.changeset/funny-cars-dance.md
@@ -0,0 +1,6 @@
+---
+"@smithy/smithy-client": minor
+"@smithy/types": minor
+---
+
+allow adding new checksum algorithms via extension
diff --git a/packages/smithy-client/package.json b/packages/smithy-client/package.json
index 9438886181b..34bdc4561a2 100644
--- a/packages/smithy-client/package.json
+++ b/packages/smithy-client/package.json
@@ -13,7 +13,9 @@
     "format": "prettier --config ../../prettier.config.js --ignore-path ../../.prettierignore --write \"**/*.{ts,md,json}\"",
     "extract:docs": "node ./scripts/fix-api-extractor && api-extractor run --local && node ./scripts/fix-api-extractor --unset",
     "test": "yarn g:vitest run",
-    "test:watch": "yarn g:vitest watch"
+    "test:watch": "yarn g:vitest watch",
+    "test:integration": "yarn g:vitest run -c vitest.config.integ.mts",
+    "test:integration:watch": "yarn g:vitest watch -c vitest.config.integ.mts"
   },
   "main": "./dist-cjs/index.js",
   "module": "./dist-es/index.js",
diff --git a/packages/smithy-client/src/extensions/checksum.integ.spec.ts b/packages/smithy-client/src/extensions/checksum.integ.spec.ts
new file mode 100644
index 00000000000..3cd4cef4bec
--- /dev/null
+++ b/packages/smithy-client/src/extensions/checksum.integ.spec.ts
@@ -0,0 +1,70 @@
+import { RpcV2Protocol } from "@smithy/smithy-rpcv2-cbor-schema";
+import type { Checksum } from "@smithy/types";
+import { describe, expect, test as it } from "vitest";
+
+import type { PartialChecksumRuntimeConfigType } from "./checksum";
+
+describe("checksum extension", () => {
+  it("should allow definition of new checksum algorithms via runtime extension", async () => {
+    class Sha256Custom implements Checksum {
+      update() {}
+      async digest() {
+        return new Uint8Array(4);
+      }
+      reset() {}
+    }
+
+    class R1 {
+      update() {}
+      async digest() {
+        return new Uint8Array(4);
+      }
+      reset() {}
+    }
+
+    const client = new RpcV2Protocol({
+      endpoint: "https://localhost",
+      extensions: [
+        {
+          configure(ext) {
+            ext.addChecksumAlgorithm({
+              algorithmId() {
+                return "r1";
+              },
+              checksumConstructor() {
+                return R1;
+              },
+            });
+            ext.addChecksumAlgorithm({
+              algorithmId() {
+                return "sha256";
+              },
+              checksumConstructor() {
+                return Sha256Custom;
+              },
+            });
+          },
+        },
+      ],
+    });
+
+    const config = client.config as typeof client.config & PartialChecksumRuntimeConfigType;
+
+    expect(config.checksumAlgorithms).toEqual({
+      // the algo id is used as the key if it is not recognized.
+      r1: R1,
+
+      // Rhe uppercase form is used if it is recognized.
+      // This matches the key in the algorithm selector function.
+      SHA256: Sha256Custom,
+    });
+
+    // for known algorithms that exist on the config, they are also set by the extension.
+    expect(config.sha256).toEqual(Sha256Custom);
+    expect(config.md5).toEqual(undefined);
+    expect(config.sha1).toEqual(undefined);
+
+    // for novel algorithms, they are not set to new fields on the config.
+    expect((config as any).r1).toEqual(undefined);
+  });
+});
diff --git a/packages/smithy-client/src/extensions/checksum.ts b/packages/smithy-client/src/extensions/checksum.ts
index 3c205c32d96..133aff40d0a 100644
--- a/packages/smithy-client/src/extensions/checksum.ts
+++ b/packages/smithy-client/src/extensions/checksum.ts
@@ -6,17 +6,24 @@ export { AlgorithmId, ChecksumAlgorithm, ChecksumConfiguration };
 /**
  * @internal
  */
-export type PartialChecksumRuntimeConfigType = Partial<{
-  sha256: ChecksumConstructor | HashConstructor;
-  md5: ChecksumConstructor | HashConstructor;
-  crc32: ChecksumConstructor | HashConstructor;
-  crc32c: ChecksumConstructor | HashConstructor;
-  sha1: ChecksumConstructor | HashConstructor;
-}>;
+const knownAlgorithms: string[] = Object.values(AlgorithmId);
 
 /**
  * @internal
  */
+export type PartialChecksumRuntimeConfigType = {
+  checksumAlgorithms?: Record<string, ChecksumConstructor | HashConstructor>;
+  sha256?: ChecksumConstructor | HashConstructor;
+  md5?: ChecksumConstructor | HashConstructor;
+  crc32?: ChecksumConstructor | HashConstructor;
+  crc32c?: ChecksumConstructor | HashConstructor;
+  sha1?: ChecksumConstructor | HashConstructor;
+};
+
+/**
+ * @param runtimeConfig - config object of the client instance.
+ * @internal
+ */
 export const getChecksumConfiguration = (runtimeConfig: PartialChecksumRuntimeConfigType) => {
   const checksumAlgorithms: ChecksumAlgorithm[] = [];
 
@@ -30,9 +37,22 @@ export const getChecksumConfiguration = (runtimeConfig: PartialChecksumRuntimeCo
       checksumConstructor: () => runtimeConfig[algorithmId]!,
     });
   }
-
+  for (const [id, ChecksumCtor] of Object.entries(runtimeConfig.checksumAlgorithms ?? {})) {
+    checksumAlgorithms.push({
+      algorithmId: () => id,
+      checksumConstructor: () => ChecksumCtor,
+    });
+  }
   return {
     addChecksumAlgorithm(algo: ChecksumAlgorithm): void {
+      runtimeConfig.checksumAlgorithms = runtimeConfig.checksumAlgorithms ?? {};
+      const id = algo.algorithmId();
+      const ctor = algo.checksumConstructor();
+      if (knownAlgorithms.includes(id)) {
+        runtimeConfig.checksumAlgorithms[id.toUpperCase()] = ctor;
+      } else {
+        runtimeConfig.checksumAlgorithms[id] = ctor;
+      }
       checksumAlgorithms.push(algo);
     },
     checksumAlgorithms(): ChecksumAlgorithm[] {
@@ -47,7 +67,11 @@ export const getChecksumConfiguration = (runtimeConfig: PartialChecksumRuntimeCo
 export const resolveChecksumRuntimeConfig = (clientConfig: ChecksumConfiguration): PartialChecksumRuntimeConfigType => {
   const runtimeConfig: PartialChecksumRuntimeConfigType = {};
   clientConfig.checksumAlgorithms().forEach((checksumAlgorithm) => {
-    runtimeConfig[checksumAlgorithm.algorithmId()] = checksumAlgorithm.checksumConstructor();
+    const id = checksumAlgorithm.algorithmId();
+    if (knownAlgorithms.includes(id)) {
+      runtimeConfig[id as AlgorithmId] = checksumAlgorithm.checksumConstructor();
+    }
+    // else the algorithm was attached to the checksumAlgorithms object on the client config already.
   });
 
   return runtimeConfig;
diff --git a/packages/smithy-client/vitest.config.integ.mts b/packages/smithy-client/vitest.config.integ.mts
new file mode 100644
index 00000000000..5802db1ac64
--- /dev/null
+++ b/packages/smithy-client/vitest.config.integ.mts
@@ -0,0 +1,8 @@
+import { defineConfig } from "vitest/config";
+
+export default defineConfig({
+  test: {
+    include: ["**/*.integ.spec.ts"],
+    environment: "node",
+  },
+});
diff --git a/packages/types/src/extensions/checksum.ts b/packages/types/src/extensions/checksum.ts
index 39c4fb20623..c8ae3c8b491 100644
--- a/packages/types/src/extensions/checksum.ts
+++ b/packages/types/src/extensions/checksum.ts
@@ -16,7 +16,7 @@ export enum AlgorithmId {
  * @internal
  */
 export interface ChecksumAlgorithm {
-  algorithmId(): AlgorithmId;
+  algorithmId(): AlgorithmId | string;
   checksumConstructor(): ChecksumConstructor | HashConstructor;
 }