diff --git a/packages/contracts-bedrock/src/L2/IL2ToL2CrossDomainMessenger.sol b/packages/contracts-bedrock/src/L2/IL2ToL2CrossDomainMessenger.sol
new file mode 100644
index 000000000000..19a627ff9a30
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+++ b/packages/contracts-bedrock/src/L2/IL2ToL2CrossDomainMessenger.sol
@@ -0,0 +1,43 @@
+// SPDX-License-Identifier: MIT
+pragma solidity ^0.8.0;
+
+/// @title IL2ToL2CrossDomainMessenger
+/// @notice Interface for the L2ToL2CrossDomainMessenger contract.
+interface IL2ToL2CrossDomainMessenger {
+    /// @notice Retrieves the sender of the current cross domain message.
+    /// @return _sender Address of the sender of the current cross domain message.
+    function crossDomainMessageSender() external view returns (address _sender);
+
+    /// @notice Retrieves the source of the current cross domain message.
+    /// @return _source Chain ID of the source of the current cross domain message.
+    function crossDomainMessageSource() external view returns (uint256 _source);
+
+    /// @notice Sends a message to some target address on a destination chain. Note that if the call
+    ///         always reverts, then the message will be unrelayable, and any ETH sent will be
+    ///         permanently locked. The same will occur if the target on the other chain is
+    ///         considered unsafe (see the _isUnsafeTarget() function).
+    /// @param _destination Chain ID of the destination chain.
+    /// @param _target      Target contract or wallet address.
+    /// @param _message     Message to trigger the target address with.
+    function sendMessage(uint256 _destination, address _target, bytes calldata _message) external payable;
+
+    /// @notice Relays a message that was sent by the other CrossDomainMessenger contract. Can only
+    ///         be executed via cross-chain call from the other messenger OR if the message was
+    ///         already received once and is currently being replayed.
+    /// @param _destination Chain ID of the destination chain.
+    /// @param _nonce       Nonce of the message being relayed.
+    /// @param _sender      Address of the user who sent the message.
+    /// @param _source      Chain ID of the source chain.
+    /// @param _target      Address that the message is targeted at.
+    /// @param _message     Message to send to the target.
+    function relayMessage(
+        uint256 _destination,
+        uint256 _source,
+        uint256 _nonce,
+        address _sender,
+        address _target,
+        bytes calldata _message
+    )
+        external
+        payable;
+}