diff --git a/packages/portal-contracts/src/Portal.sol b/packages/portal-contracts/src/Portal.sol
index 41f4770..e61aba9 100644
--- a/packages/portal-contracts/src/Portal.sol
+++ b/packages/portal-contracts/src/Portal.sol
@@ -78,6 +78,13 @@ contract Portal is Owned {
         address maker
     );
 
+    event OrderUpdated(
+        uint256 orderID,
+        int128 amountSats,
+        uint256 makerStakedTok,
+        address maker
+    );
+
     event OrderCancelled(uint256 orderID);
 
     event OrderMatched(
@@ -414,6 +421,42 @@ contract Portal is Owned {
         _transferToSender(tokToSend);
     }
 
+    // Update an existing bid or ask order. amountSats is negative if the order is an ask.
+    function updateOrder(uint256 orderID, int128 amountSats) public payable {
+        Order storage o = orderbook[orderID];
+
+        require(o.amountSats != 0, "Order not found");
+        require(o.maker == msg.sender, "Order not yours");
+
+        if (o.amountSats < 0) {
+          // This is an ask. Receive the ether to be sold and update the order.
+          require(amountSats < 0 && uint256(int256(-(o.amountSats + amountSats))) <= MAX_SATS, "Must be adding a valid amount of liquidity");
+
+          uint256 additionalValue = uint256(int256(-amountSats * int128(o.priceTokPerSat)));
+          o.amountSats += amountSats;
+
+          _transferFromSender(additionalValue);
+        } else {
+          // This is a bid, update the order and take the additional stake
+          require(amountSats > 0 && uint256(int256(o.amountSats)) + uint256(int256(amountSats)) <= MAX_SATS, "Must be adding a valid amount of liquidity");
+          uint256 expectedStakeTok = (uint256(int256(amountSats)) * uint256(o.priceTokPerSat) * stakePercent) / 100;
+          
+          // Update the order
+          o.amountSats += amountSats;
+          o.stakedTok += expectedStakeTok;
+
+          // Receive the additional stake
+          _transferFromSender(expectedStakeTok);
+        }
+
+        emit OrderUpdated(
+          orderID,
+          o.amountSats,
+          o.stakedTok,
+          o.maker
+        );
+    }
+
     function _transferFromSender(uint256 tok) private {
         if (address(token) == address(0)) {
             // Receive wei
diff --git a/packages/portal-contracts/src/test/Portal.t.sol b/packages/portal-contracts/src/test/Portal.t.sol
index a78d8d3..6a009bf 100644
--- a/packages/portal-contracts/src/test/Portal.t.sol
+++ b/packages/portal-contracts/src/test/Portal.t.sol
@@ -13,6 +13,13 @@ contract PortalTest is Test {
         uint256 makerStakedWei,
         address maker
     );
+    
+    event OrderUpdated(
+        uint256 orderID,
+        int128 amountSats,
+        uint256 makerStakedTok,
+        address maker
+    );
 
     event OrderCancelled(uint256 orderID);
 
@@ -360,6 +367,52 @@ contract PortalTest is Test {
         emit EscrowSettled(2, 1e8-1e7, address(this), 18.9 ether);
         p.proveSettlement(2, 123, proof, 12);
     }
+    
+    function testUpdateOrderNotMaker() public {
+        Portal p = testBid();
+        vm.prank(address(0x42));
+        vm.expectRevert(bytes("Order not yours"));
+        p.updateOrder(1, 1e8);
+    }
+
+    function testUpdateBid() public {
+        Portal p = testBid();
+        
+        uint256 stakeWei = 1 ether; /* 1 ETH = 5% of 1 * 20 ETH */
+        vm.expectEmit(true, true, true, true);
+        emit OrderUpdated(1, 2e8, 2e18, address(this));
+        p.updateOrder{value: stakeWei}(1, 1e8);
+        
+        uint256 orderID = 1;
+        bytes20 destScriptHash = hex"0011223344556677889900112233445566778899";
+        uint256 escrowID = p.initiateSell{value: 40 ether}(
+            orderID,
+            2e8,
+            destScriptHash
+        );
+        assertEq(escrowID, 1);
+        
+        vm.expectRevert(bytes("Order already filled"));
+        p.initiateSell{value: 20 ether}(orderID, 1e8, destScriptHash);
+    }
+    
+    function testUpdateAsk() public {
+        Portal p = testAsk();
+        
+        vm.expectEmit(true, true, true, true);
+        emit OrderUpdated(1, -2e8, 0, address(this));
+        p.updateOrder{value: 20 ether}(1, -1e8);
+        
+        uint256 orderID = 1;
+        uint256 escrowID = p.initiateBuy{value: 2 ether}(
+            orderID,
+            2e8
+        );
+        assertEq(escrowID, 1);
+        
+        vm.expectRevert(bytes("Order already filled"));
+        p.initiateBuy{value: 20 ether}(orderID, 1e8);
+    }
 }
 
 contract StubBtcTxVerifier is IBtcTxVerifier {