diff --git a/packages/eth-json-rpc-middleware/CHANGELOG.md b/packages/eth-json-rpc-middleware/CHANGELOG.md
index f010b3086f..1995befb4d 100644
--- a/packages/eth-json-rpc-middleware/CHANGELOG.md
+++ b/packages/eth-json-rpc-middleware/CHANGELOG.md
@@ -11,6 +11,12 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 - Bump `@metamask/superstruct` from `^3.1.0` to `^3.4.1` ([#9754](https://github.com/MetaMask/core/pull/9754))
 
+### Fixed
+
+- Accept a numeric `chainId` in `eth_sendTransaction` and `eth_signTransaction` params ([#9965](https://github.com/MetaMask/core/pull/9965))
+  - `chainId` is now validated as a quantity (hex string or number), matching `gas`, `nonce`, `value`, and the other numerical fields
+  - Previously it was restricted to a string, so dapps sending a number were rejected with `-32602 Invalid params`
+
 ## [24.0.0]
 
 ### Changed
diff --git a/packages/eth-json-rpc-middleware/src/utils/validation.test.ts b/packages/eth-json-rpc-middleware/src/utils/validation.test.ts
index 2d4e343a4e..b3aa4d8b08 100644
--- a/packages/eth-json-rpc-middleware/src/utils/validation.test.ts
+++ b/packages/eth-json-rpc-middleware/src/utils/validation.test.ts
@@ -333,6 +333,37 @@ describe('Validation Utils', () => {
       ).not.toThrow();
     });
 
+    it('does not throw when `chainId` is a number', () => {
+      expect(() =>
+        validateTransactionParams({
+          from: VALID_FROM,
+          to: VALID_TO,
+          chainId: 1,
+        }),
+      ).not.toThrow();
+    });
+
+    it('does not throw when `chainId` is a hex string', () => {
+      expect(() =>
+        validateTransactionParams({
+          from: VALID_FROM,
+          to: VALID_TO,
+          chainId: '0x1',
+        }),
+      ).not.toThrow();
+    });
+
+    it('throws for an extraneous top-level key alongside a numeric `chainId`', () => {
+      expect(() =>
+        validateTransactionParams({
+          from: VALID_FROM,
+          to: VALID_TO,
+          chainId: 1,
+          extraKey: 'unexpected',
+        }),
+      ).toThrow(/Invalid params/u);
+    });
+
     it.each([
       ['null', null],
       ['undefined', undefined],
diff --git a/packages/eth-json-rpc-middleware/src/utils/validation.ts b/packages/eth-json-rpc-middleware/src/utils/validation.ts
index 4c68d0ed80..81662ac3ad 100644
--- a/packages/eth-json-rpc-middleware/src/utils/validation.ts
+++ b/packages/eth-json-rpc-middleware/src/utils/validation.ts
@@ -263,7 +263,7 @@ export const TransactionParamsStruct = object({
       }),
     ),
   ),
-  chainId: optional(string()),
+  chainId: optional(QuantityStruct),
   data: optional(string()),
   from: string(),
   gas: optional(QuantityStruct),