diff --git a/packages/api/src/rpc/eth/eth.js b/packages/api/src/rpc/eth/eth.js
index 3263f32b..6aefd42d 100644
--- a/packages/api/src/rpc/eth/eth.js
+++ b/packages/api/src/rpc/eth/eth.js
@@ -39,6 +39,12 @@ class Eth {
       .send('eth_call', inOptions(options), inBlockNumber(blockNumber));
   }
 
+  chainId () {
+    return this._provider
+      .send('eth_chainId')
+      .then(outNumber);
+  }
+
   coinbase () {
     return this._provider
       .send('eth_coinbase')
diff --git a/packages/light.js/src/index.ts b/packages/light.js/src/index.ts
index 83893e2d..0d87fc52 100644
--- a/packages/light.js/src/index.ts
+++ b/packages/light.js/src/index.ts
@@ -17,6 +17,7 @@ export const {
   accounts$,
   accountsInfo$,
   balanceOf$,
+  chainId$,
   transactionCountOf$,
   blockNumber$,
   chainName$,
diff --git a/packages/light.js/src/rpc/eth.ts b/packages/light.js/src/rpc/eth.ts
index 7733d259..21b736c6 100644
--- a/packages/light.js/src/rpc/eth.ts
+++ b/packages/light.js/src/rpc/eth.ts
@@ -13,6 +13,22 @@ import frequency from '../frequency';
 import { isNullOrLoading, RPC_LOADING } from '../utils/isLoading';
 import { switchMapPromise } from '../utils/operators';
 
+/**
+ * Observable containing the EIP155 chain ID used for transaction signing.
+ * Calls `eth_chainId`
+ *
+ * @param options - Options to pass to {@link RpcObservableOptions}.
+ * @return - An Observable containing the chain ID.
+ */
+export function chainId$ (options?: RpcObservableOptions) {
+  return createRpc$<any, BigNumber | Symbol>({
+    calls: ['eth_chainId'],
+    frequency: [frequency.onStartup$],
+    name: 'chainId$',
+    pipes: api => [switchMapPromise(() => api.eth.chainId())]
+  })(options)();
+}
+
 /**
  * Observable which contains the array of all addresses managed by the light
  * client.
diff --git a/packages/light.js/src/utils/testHelpers/mockApi.ts b/packages/light.js/src/utils/testHelpers/mockApi.ts
index 341780c3..868b2ccd 100644
--- a/packages/light.js/src/utils/testHelpers/mockApi.ts
+++ b/packages/light.js/src/utils/testHelpers/mockApi.ts
@@ -22,7 +22,7 @@ export class MockProvider extends EventEmitter {
 
 // List of JSONRPCs we want to mock
 const listOfMockRps: { [index: string]: string[] } = {
-  eth: ['accounts', 'blockNumber', 'getBalance', 'getTransactionCount', 'newHeads', 'syncing'],
+  eth: ['accounts', 'blockNumber', 'chainId', 'getBalance', 'getTransactionCount', 'newHeads', 'syncing'],
   fake: ['method'],
   net: ['peerCount'],
   parity: ['accountsInfo', 'chain', 'postTransaction']