diff --git a/packages/network-enablement-controller/CHANGELOG.md b/packages/network-enablement-controller/CHANGELOG.md
index 1da86595dc..be56e6dfe1 100644
--- a/packages/network-enablement-controller/CHANGELOG.md
+++ b/packages/network-enablement-controller/CHANGELOG.md
@@ -7,6 +7,10 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## [Unreleased]
 
+### Fixed
+
+- Remove Sei Mainnet (`0x531`) from the default-enabled EVM network map, fixing a crash in consumers that call `findNetworkClientIdByChainId` for chains with no built-in `NetworkController` client ([#9542](https://github.com/MetaMask/core/pull/9542))
+
 ## [5.6.0]
 
 ### Added
diff --git a/packages/network-enablement-controller/src/NetworkEnablementController.test.ts b/packages/network-enablement-controller/src/NetworkEnablementController.test.ts
index 410e68a089..07e23531db 100644
--- a/packages/network-enablement-controller/src/NetworkEnablementController.test.ts
+++ b/packages/network-enablement-controller/src/NetworkEnablementController.test.ts
@@ -195,7 +195,6 @@ describe('NetworkEnablementController', () => {
           [ChainId[BuiltInNetworkName.BscMainnet]]: true,
           [ChainId[BuiltInNetworkName.OptimismMainnet]]: true,
           [ChainId[BuiltInNetworkName.PolygonMainnet]]: true,
-          [ChainId[BuiltInNetworkName.SeiMainnet]]: true,
           [ChainId[BuiltInNetworkName.MonadMainnet]]: true,
         },
         [KnownCaipNamespace.Solana]: {
@@ -255,7 +254,6 @@ describe('NetworkEnablementController', () => {
           [ChainId[BuiltInNetworkName.BscMainnet]]: true,
           [ChainId[BuiltInNetworkName.OptimismMainnet]]: true,
           [ChainId[BuiltInNetworkName.PolygonMainnet]]: true,
-          [ChainId[BuiltInNetworkName.SeiMainnet]]: true,
           [ChainId[BuiltInNetworkName.MonadMainnet]]: true,
           '0xa86a': true, // Avalanche network added and enabled (keeps current selection)
         },
@@ -324,7 +322,6 @@ describe('NetworkEnablementController', () => {
           [ChainId[BuiltInNetworkName.BscMainnet]]: true,
           [ChainId[BuiltInNetworkName.OptimismMainnet]]: true,
           [ChainId[BuiltInNetworkName.PolygonMainnet]]: true,
-          [ChainId[BuiltInNetworkName.SeiMainnet]]: true,
           [ChainId[BuiltInNetworkName.MonadMainnet]]: true,
         },
         [KnownCaipNamespace.Solana]: {
@@ -433,7 +430,6 @@ describe('NetworkEnablementController', () => {
     controller.disableNetwork('0x38'); // BSC Mainnet
     controller.disableNetwork('0xa'); // Optimism Mainnet
     controller.disableNetwork('0x89'); // Polygon Mainnet
-    controller.disableNetwork('0x531'); // Sei Mainnet
     controller.disableNetwork('0x8f'); // Monad Mainnet
 
     // Publish an update with linea network removed
@@ -471,7 +467,6 @@ describe('NetworkEnablementController', () => {
           [ChainId[BuiltInNetworkName.BscMainnet]]: false,
           [ChainId[BuiltInNetworkName.OptimismMainnet]]: false,
           [ChainId[BuiltInNetworkName.PolygonMainnet]]: false,
-          [ChainId[BuiltInNetworkName.SeiMainnet]]: false,
           [ChainId[BuiltInNetworkName.MonadMainnet]]: false,
         },
         [KnownCaipNamespace.Solana]: {
@@ -576,7 +571,6 @@ describe('NetworkEnablementController', () => {
             [ChainId[BuiltInNetworkName.BscMainnet]]: true,
             [ChainId[BuiltInNetworkName.OptimismMainnet]]: true,
             [ChainId[BuiltInNetworkName.PolygonMainnet]]: true,
-            [ChainId[BuiltInNetworkName.SeiMainnet]]: true,
             [ChainId[BuiltInNetworkName.MonadMainnet]]: true,
           },
           [KnownCaipNamespace.Solana]: {
@@ -1227,7 +1221,6 @@ describe('NetworkEnablementController', () => {
             [ChainId[BuiltInNetworkName.BscMainnet]]: true,
             [ChainId[BuiltInNetworkName.OptimismMainnet]]: true,
             [ChainId[BuiltInNetworkName.PolygonMainnet]]: true,
-            [ChainId[BuiltInNetworkName.SeiMainnet]]: true,
             [ChainId[BuiltInNetworkName.MonadMainnet]]: true,
           },
           [KnownCaipNamespace.Solana]: {
@@ -1266,7 +1259,6 @@ describe('NetworkEnablementController', () => {
             [ChainId[BuiltInNetworkName.BscMainnet]]: false, // Not in mocked config
             [ChainId[BuiltInNetworkName.OptimismMainnet]]: false, // Not in mocked config
             [ChainId[BuiltInNetworkName.PolygonMainnet]]: false, // Not in mocked config
-            [ChainId[BuiltInNetworkName.SeiMainnet]]: false, // Not in mocked config
             [ChainId[BuiltInNetworkName.MonadMainnet]]: false, // Not in mocked config
           },
           [KnownCaipNamespace.Solana]: {
@@ -1538,7 +1530,6 @@ describe('NetworkEnablementController', () => {
             [ChainId[BuiltInNetworkName.BscMainnet]]: true,
             [ChainId[BuiltInNetworkName.OptimismMainnet]]: true,
             [ChainId[BuiltInNetworkName.PolygonMainnet]]: true,
-            [ChainId[BuiltInNetworkName.SeiMainnet]]: true,
             [ChainId[BuiltInNetworkName.MonadMainnet]]: true,
           },
           [KnownCaipNamespace.Solana]: {
@@ -1577,7 +1568,6 @@ describe('NetworkEnablementController', () => {
             [ChainId[BuiltInNetworkName.BscMainnet]]: false,
             [ChainId[BuiltInNetworkName.OptimismMainnet]]: false,
             [ChainId[BuiltInNetworkName.PolygonMainnet]]: false,
-            [ChainId[BuiltInNetworkName.SeiMainnet]]: false,
             [ChainId[BuiltInNetworkName.MonadMainnet]]: false,
           },
           [KnownCaipNamespace.Solana]: {
@@ -1636,7 +1626,6 @@ describe('NetworkEnablementController', () => {
             [ChainId[BuiltInNetworkName.BscMainnet]]: false,
             [ChainId[BuiltInNetworkName.OptimismMainnet]]: false,
             [ChainId[BuiltInNetworkName.PolygonMainnet]]: false,
-            [ChainId[BuiltInNetworkName.SeiMainnet]]: false,
             [ChainId[BuiltInNetworkName.MonadMainnet]]: false,
           },
           [KnownCaipNamespace.Solana]: {
@@ -1679,7 +1668,6 @@ describe('NetworkEnablementController', () => {
             [ChainId[BuiltInNetworkName.BscMainnet]]: false,
             [ChainId[BuiltInNetworkName.OptimismMainnet]]: false,
             [ChainId[BuiltInNetworkName.PolygonMainnet]]: false,
-            [ChainId[BuiltInNetworkName.SeiMainnet]]: false,
             [ChainId[BuiltInNetworkName.MonadMainnet]]: false,
           },
           [KnownCaipNamespace.Solana]: {
@@ -1722,7 +1710,6 @@ describe('NetworkEnablementController', () => {
             [ChainId[BuiltInNetworkName.BscMainnet]]: false,
             [ChainId[BuiltInNetworkName.OptimismMainnet]]: false,
             [ChainId[BuiltInNetworkName.PolygonMainnet]]: false,
-            [ChainId[BuiltInNetworkName.SeiMainnet]]: false,
             [ChainId[BuiltInNetworkName.MonadMainnet]]: false,
           },
           [KnownCaipNamespace.Solana]: {
@@ -1776,7 +1763,6 @@ describe('NetworkEnablementController', () => {
             [ChainId[BuiltInNetworkName.BscMainnet]]: false,
             [ChainId[BuiltInNetworkName.OptimismMainnet]]: false,
             [ChainId[BuiltInNetworkName.PolygonMainnet]]: false,
-            [ChainId[BuiltInNetworkName.SeiMainnet]]: false,
             [ChainId[BuiltInNetworkName.MonadMainnet]]: false,
           },
           [KnownCaipNamespace.Solana]: {
@@ -1827,7 +1813,6 @@ describe('NetworkEnablementController', () => {
             [ChainId[BuiltInNetworkName.BscMainnet]]: false,
             [ChainId[BuiltInNetworkName.OptimismMainnet]]: false,
             [ChainId[BuiltInNetworkName.PolygonMainnet]]: false,
-            [ChainId[BuiltInNetworkName.SeiMainnet]]: false,
             [ChainId[BuiltInNetworkName.MonadMainnet]]: false,
           },
           [KnownCaipNamespace.Solana]: {
@@ -1881,7 +1866,6 @@ describe('NetworkEnablementController', () => {
             [ChainId[BuiltInNetworkName.BscMainnet]]: false,
             [ChainId[BuiltInNetworkName.OptimismMainnet]]: false,
             [ChainId[BuiltInNetworkName.PolygonMainnet]]: false,
-            [ChainId[BuiltInNetworkName.SeiMainnet]]: false,
             [ChainId[BuiltInNetworkName.MonadMainnet]]: false,
           },
           [KnownCaipNamespace.Solana]: {
@@ -1932,7 +1916,6 @@ describe('NetworkEnablementController', () => {
             [ChainId[BuiltInNetworkName.BscMainnet]]: true,
             [ChainId[BuiltInNetworkName.OptimismMainnet]]: true,
             [ChainId[BuiltInNetworkName.PolygonMainnet]]: true,
-            [ChainId[BuiltInNetworkName.SeiMainnet]]: true,
             [ChainId[BuiltInNetworkName.MonadMainnet]]: true,
           },
           [KnownCaipNamespace.Solana]: {
@@ -1985,7 +1968,6 @@ describe('NetworkEnablementController', () => {
             [ChainId[BuiltInNetworkName.BscMainnet]]: true,
             [ChainId[BuiltInNetworkName.OptimismMainnet]]: true,
             [ChainId[BuiltInNetworkName.PolygonMainnet]]: true,
-            [ChainId[BuiltInNetworkName.SeiMainnet]]: true,
             [ChainId[BuiltInNetworkName.MonadMainnet]]: true,
           },
           [KnownCaipNamespace.Solana]: {
diff --git a/packages/network-enablement-controller/src/NetworkEnablementController.ts b/packages/network-enablement-controller/src/NetworkEnablementController.ts
index 844955887c..9aa833d015 100644
--- a/packages/network-enablement-controller/src/NetworkEnablementController.ts
+++ b/packages/network-enablement-controller/src/NetworkEnablementController.ts
@@ -158,6 +158,9 @@ export type NetworkConfig = {
 const getDefaultNetworkEnablementControllerState =
   (): NetworkEnablementControllerState => ({
     enabledNetworkMap: {
+      // Each chain ID here must have a built-in client in NetworkController
+      // (see DEFAULT_INFURA_NETWORKS). A missing client causes
+      // findNetworkClientIdByChainId to throw silently in consumers.
       [KnownCaipNamespace.Eip155]: {
         [ChainId[BuiltInNetworkName.Mainnet]]: true,
         [ChainId[BuiltInNetworkName.LineaMainnet]]: true,
@@ -166,7 +169,6 @@ const getDefaultNetworkEnablementControllerState =
         [ChainId[BuiltInNetworkName.BscMainnet]]: true,
         [ChainId[BuiltInNetworkName.OptimismMainnet]]: true,
         [ChainId[BuiltInNetworkName.PolygonMainnet]]: true,
-        [ChainId[BuiltInNetworkName.SeiMainnet]]: true,
         [ChainId[BuiltInNetworkName.MonadMainnet]]: true,
       },
       [KnownCaipNamespace.Solana]: {