diff --git a/.changeset/famous-pans-act.md b/.changeset/famous-pans-act.md
new file mode 100644
index 0000000000..602ffe70ac
--- /dev/null
+++ b/.changeset/famous-pans-act.md
@@ -0,0 +1,5 @@
+---
+"viem": patch
+---
+
+Add `blockTime` to Arbitrum chains.
diff --git a/src/chains/definitions/arbitrum.ts b/src/chains/definitions/arbitrum.ts
index 624e9f5885..a8eee7cd0b 100644
--- a/src/chains/definitions/arbitrum.ts
+++ b/src/chains/definitions/arbitrum.ts
@@ -4,6 +4,7 @@ export const arbitrum = /*#__PURE__*/ defineChain({
   id: 42_161,
   name: 'Arbitrum One',
   nativeCurrency: { name: 'Ether', symbol: 'ETH', decimals: 18 },
+  blockTime: 250,
   rpcUrls: {
     default: {
       http: ['https://arb1.arbitrum.io/rpc'],
diff --git a/src/chains/definitions/arbitrumSepolia.ts b/src/chains/definitions/arbitrumSepolia.ts
index 5a923ecdf7..dbfd20e165 100644
--- a/src/chains/definitions/arbitrumSepolia.ts
+++ b/src/chains/definitions/arbitrumSepolia.ts
@@ -3,6 +3,7 @@ import { defineChain } from '../../utils/chain/defineChain.js'
 export const arbitrumSepolia = /*#__PURE__*/ defineChain({
   id: 421_614,
   name: 'Arbitrum Sepolia',
+  blockTime: 250,
   nativeCurrency: {
     name: 'Arbitrum Sepolia Ether',
     symbol: 'ETH',