diff --git a/crates/optimism/chainspec/src/lib.rs b/crates/optimism/chainspec/src/lib.rs
index 0ee86bc7d24b..907599fe2a29 100644
--- a/crates/optimism/chainspec/src/lib.rs
+++ b/crates/optimism/chainspec/src/lib.rs
@@ -36,7 +36,7 @@ use reth_chainspec::{
 };
 use reth_ethereum_forks::{ChainHardforks, EthereumHardfork, ForkCondition, Hardfork};
 use reth_network_peers::NodeRecord;
-use reth_optimism_forks::OpHardforks;
+use reth_optimism_forks::{OpHardfork, OpHardforks};
 #[cfg(feature = "std")]
 pub(crate) use std::sync::LazyLock;
 
@@ -166,6 +166,13 @@ impl OpChainSpecBuilder {
         self
     }
 
+    /// Enable Isthmus at genesis
+    pub fn isthmus_activated(mut self) -> Self {
+        self = self.holocene_activated();
+        self.inner = self.inner.with_fork(OpHardfork::Isthmus, ForkCondition::Timestamp(0));
+        self
+    }
+
     /// Build the resulting [`OpChainSpec`].
     ///
     /// # Panics
@@ -414,6 +421,7 @@ impl From<Genesis> for OpChainSpec {
             (OpHardfork::Fjord.boxed(), genesis_info.fjord_time),
             (OpHardfork::Granite.boxed(), genesis_info.granite_time),
             (OpHardfork::Holocene.boxed(), genesis_info.holocene_time),
+            (OpHardfork::Isthmus.boxed(), genesis_info.isthmus_time),
         ];
 
         let mut time_hardforks = time_hardfork_opts
@@ -1030,6 +1038,7 @@ mod tests {
             OpHardfork::Fjord.boxed(),
             OpHardfork::Granite.boxed(),
             OpHardfork::Holocene.boxed(),
+            // OpHardfork::Isthmus.boxed(),
         ];
 
         assert!(expected_hardforks
diff --git a/crates/optimism/evm/src/config.rs b/crates/optimism/evm/src/config.rs
index 4a37860efc6b..b32b0929424d 100644
--- a/crates/optimism/evm/src/config.rs
+++ b/crates/optimism/evm/src/config.rs
@@ -12,7 +12,9 @@ pub fn revm_spec_by_timestamp_after_bedrock(
     chain_spec: &OpChainSpec,
     timestamp: u64,
 ) -> revm_primitives::SpecId {
-    if chain_spec.fork(OpHardfork::Holocene).active_at_timestamp(timestamp) {
+    if chain_spec.fork(OpHardfork::Isthmus).active_at_timestamp(timestamp) {
+        todo!()
+    } else if chain_spec.fork(OpHardfork::Holocene).active_at_timestamp(timestamp) {
         revm_primitives::HOLOCENE
     } else if chain_spec.fork(OpHardfork::Granite).active_at_timestamp(timestamp) {
         revm_primitives::GRANITE
@@ -31,7 +33,9 @@ pub fn revm_spec_by_timestamp_after_bedrock(
 
 /// Map the latest active hardfork at the given block to a revm [`SpecId`](revm_primitives::SpecId).
 pub fn revm_spec(chain_spec: &OpChainSpec, block: &Head) -> revm_primitives::SpecId {
-    if chain_spec.fork(OpHardfork::Holocene).active_at_head(block) {
+    if chain_spec.fork(OpHardfork::Isthmus).active_at_head(block) {
+        todo!()
+    } else if chain_spec.fork(OpHardfork::Holocene).active_at_head(block) {
         revm_primitives::HOLOCENE
     } else if chain_spec.fork(OpHardfork::Granite).active_at_head(block) {
         revm_primitives::GRANITE
diff --git a/crates/optimism/hardforks/src/hardfork.rs b/crates/optimism/hardforks/src/hardfork.rs
index 661816ae5fe0..962d7bca4bcd 100644
--- a/crates/optimism/hardforks/src/hardfork.rs
+++ b/crates/optimism/hardforks/src/hardfork.rs
@@ -33,6 +33,8 @@ hardfork!(
         Granite,
         /// Holocene: <https://github.com/ethereum-optimism/specs/blob/main/specs/protocol/superchain-upgrades.md#holocene>
         Holocene,
+        /// Isthmus: <https://github.com/ethereum-optimism/specs/blob/main/specs/protocol/isthmus/overview.md>
+        Isthmus,
     }
 );
 
@@ -159,6 +161,7 @@ impl OpHardfork {
                 Self::Fjord => Some(1716998400),
                 Self::Granite => Some(1723478400),
                 Self::Holocene => Some(1732633200),
+                Self::Isthmus => todo!(),
             },
         )
     }
@@ -194,6 +197,7 @@ impl OpHardfork {
                 Self::Fjord => Some(1720627201),
                 Self::Granite => Some(1726070401),
                 Self::Holocene => None,
+                Self::Isthmus => todo!(),
             },
         )
     }
@@ -357,7 +361,7 @@ mod tests {
     #[test]
     fn check_op_hardfork_from_str() {
         let hardfork_str =
-            ["beDrOck", "rEgOlITH", "cAnYoN", "eCoToNe", "FJorD", "GRaNiTe", "hOlOcEnE"];
+            ["beDrOck", "rEgOlITH", "cAnYoN", "eCoToNe", "FJorD", "GRaNiTe", "hOlOcEnE", "isthMUS"];
         let expected_hardforks = [
             OpHardfork::Bedrock,
             OpHardfork::Regolith,
@@ -366,6 +370,7 @@ mod tests {
             OpHardfork::Fjord,
             OpHardfork::Granite,
             OpHardfork::Holocene,
+            OpHardfork::Isthmus,
         ];
 
         let hardforks: Vec<OpHardfork> =
diff --git a/crates/optimism/hardforks/src/lib.rs b/crates/optimism/hardforks/src/lib.rs
index bf6ca98ce4e9..36f42155e942 100644
--- a/crates/optimism/hardforks/src/lib.rs
+++ b/crates/optimism/hardforks/src/lib.rs
@@ -27,6 +27,12 @@ pub trait OpHardforks: EthereumHardforks {
         self.fork(OpHardfork::Bedrock).active_at_block(block_number)
     }
 
+    /// Returns `true` if [`Regolith`](OpHardfork::Regolith) is active at given block
+    /// timestamp.
+    fn is_regolith_active_at_timestamp(&self, timestamp: u64) -> bool {
+        self.fork(OpHardfork::Regolith).active_at_timestamp(timestamp)
+    }
+
     /// Returns `true` if [`Canyon`](OpHardfork::Canyon) is active at given block timestamp.
     fn is_canyon_active_at_timestamp(&self, timestamp: u64) -> bool {
         self.fork(OpHardfork::Canyon).active_at_timestamp(timestamp)
@@ -53,9 +59,9 @@ pub trait OpHardforks: EthereumHardforks {
         self.fork(OpHardfork::Holocene).active_at_timestamp(timestamp)
     }
 
-    /// Returns `true` if [`Regolith`](OpHardfork::Regolith) is active at given block
+    /// Returns `true` if [`Isthmus`](OpHardfork::Isthmus) is active at given block
     /// timestamp.
-    fn is_regolith_active_at_timestamp(&self, timestamp: u64) -> bool {
-        self.fork(OpHardfork::Regolith).active_at_timestamp(timestamp)
+    fn is_isthmus_active_at_timestamp(&self, timestamp: u64) -> bool {
+        self.fork(OpHardfork::Isthmus).active_at_timestamp(timestamp)
     }
 }