diff --git a/packages/mesh-contract/src/marketplace/offchain.ts b/packages/mesh-contract/src/marketplace/offchain.ts
index f7c5c4b39..77c9f742d 100644
--- a/packages/mesh-contract/src/marketplace/offchain.ts
+++ b/packages/mesh-contract/src/marketplace/offchain.ts
@@ -24,6 +24,7 @@ import {
 import { applyParamsToScript } from "@meshsdk/core-cst";
 
 import { MeshTxInitiator, MeshTxInitiatorInput } from "../common";
+import { computeMarketplacePurchasePayouts } from "./purchase-fee-math";
 import blueprintV1 from "./aiken-workspace-v1/plutus.json";
 import blueprintV2 from "./aiken-workspace-v2/plutus.json";
 
@@ -178,27 +179,24 @@ export class MeshMarketplaceContract extends MeshTxInitiator {
       )
       .selectUtxosFrom(utxos);
 
-    let ownerToReceiveLovelace =
-      ((inputDatum.fields[1].int as number) * this.feePercentageBasisPoint) /
-      10000;
-    if (this.feePercentageBasisPoint > 0 && ownerToReceiveLovelace < 1000000) {
-      ownerToReceiveLovelace = 1000000;
-    }
+    const { ownerToReceiveLovelace, sellerToReceiveLovelace } =
+      computeMarketplacePurchasePayouts(
+        inputDatum.fields[1].int,
+        this.feePercentageBasisPoint,
+        inputLovelace,
+      );
 
-    if (ownerToReceiveLovelace > 0) {
+    if (ownerToReceiveLovelace > 0n) {
       const ownerToReceive = [
         {
           unit: "lovelace",
-          quantity: Math.ceil(ownerToReceiveLovelace).toString(),
+          quantity: ownerToReceiveLovelace.toString(),
         },
       ];
       tx.txOut(this.ownerAddress, ownerToReceive);
     }
 
-    const sellerToReceiveLovelace =
-      (inputDatum.fields[1].int as number) + Number(inputLovelace);
-
-    if (sellerToReceiveLovelace > 0) {
+    if (sellerToReceiveLovelace > 0n) {
       const sellerAddress = serializeAddressObj(
         inputDatum.fields[0],
         this.networkId,
diff --git a/packages/mesh-contract/src/marketplace/purchase-fee-math.node.test.mjs b/packages/mesh-contract/src/marketplace/purchase-fee-math.node.test.mjs
new file mode 100644
index 000000000..1b5fde61b
--- /dev/null
+++ b/packages/mesh-contract/src/marketplace/purchase-fee-math.node.test.mjs
@@ -0,0 +1,54 @@
+/**
+ * Zero-dep regression harness for marketplace purchaseAsset BigInt fee math.
+ * Run: node --test packages/mesh-contract/src/marketplace/purchase-fee-math.node.test.mjs
+ * Full helper assertions: packages/mesh-contract/src/marketplace/purchase-fee-math.test.ts (jest)
+ */
+import { describe, it } from "node:test";
+import assert from "node:assert/strict";
+import { readFileSync } from "node:fs";
+import { fileURLToPath } from "node:url";
+import { dirname, join } from "node:path";
+
+const __dirname = dirname(fileURLToPath(import.meta.url));
+
+/** Prior buggy formula from purchaseAsset. */
+function buggyLegacyMix(priceInt, feePercentageBasisPoint, inputLovelace) {
+  let ownerToReceiveLovelace =
+    (priceInt * feePercentageBasisPoint) / 10000;
+  if (feePercentageBasisPoint > 0 && ownerToReceiveLovelace < 1000000) {
+    ownerToReceiveLovelace = 1000000;
+  }
+  const sellerToReceiveLovelace = priceInt + Number(inputLovelace);
+  return { ownerToReceiveLovelace, sellerToReceiveLovelace };
+}
+
+describe("purchaseAsset fee math BigInt regression", () => {
+  it("legacy Number-mix throws TypeError when price is BigInt", () => {
+    assert.throws(
+      () => buggyLegacyMix(100_000_000n, 250, "2000000"),
+      (err) =>
+        err instanceof TypeError &&
+        /Cannot mix BigInt and other types/.test(err.message),
+    );
+  });
+
+  it("offchain.ts purchaseAsset uses computeMarketplacePurchasePayouts", () => {
+    const src = readFileSync(join(__dirname, "offchain.ts"), "utf8");
+    assert.match(src, /computeMarketplacePurchasePayouts/);
+    assert.doesNotMatch(
+      src,
+      /\(inputDatum\.fields\[1\]\.int as number\) \* this\.feePercentageBasisPoint/,
+    );
+    assert.doesNotMatch(
+      src,
+      /\(inputDatum\.fields\[1\]\.int as number\) \+ Number\(inputLovelace\)/,
+    );
+  });
+
+  it("purchase-fee-math.ts uses BigInt arithmetic", () => {
+    const src = readFileSync(join(__dirname, "purchase-fee-math.ts"), "utf8");
+    assert.match(src, /BigInt\(priceInt\)/);
+    assert.match(src, /10000n/);
+    assert.doesNotMatch(src, /as number\) \* feePercentageBasisPoint/);
+  });
+});
diff --git a/packages/mesh-contract/src/marketplace/purchase-fee-math.test.ts b/packages/mesh-contract/src/marketplace/purchase-fee-math.test.ts
new file mode 100644
index 000000000..1c71558b1
--- /dev/null
+++ b/packages/mesh-contract/src/marketplace/purchase-fee-math.test.ts
@@ -0,0 +1,34 @@
+import { describe, expect, it } from "@jest/globals";
+import { computeMarketplacePurchasePayouts } from "./purchase-fee-math";
+
+describe("computeMarketplacePurchasePayouts", () => {
+  it("does not throw TypeError when datum price int is BigInt", () => {
+    expect(() =>
+      computeMarketplacePurchasePayouts(100_000_000n, 250, "2000000"),
+    ).not.toThrow();
+  });
+
+  it("computes owner fee (2.5%) and seller payout with BigInt price", () => {
+    // 100 ADA price, 2.5% fee => 2.5 ADA (above 1 ADA minimum)
+    const { ownerToReceiveLovelace, sellerToReceiveLovelace } =
+      computeMarketplacePurchasePayouts(100_000_000n, 250, "2000000");
+    expect(ownerToReceiveLovelace).toBe(2_500_000n);
+    expect(sellerToReceiveLovelace).toBe(102_000_000n);
+  });
+
+  it("enforces minimum 1 ADA owner fee when basis points > 0", () => {
+    const { ownerToReceiveLovelace } = computeMarketplacePurchasePayouts(
+      1_000n,
+      250,
+      "0",
+    );
+    expect(ownerToReceiveLovelace).toBe(1_000_000n);
+  });
+
+  it("still works when price int is a Number", () => {
+    const { ownerToReceiveLovelace, sellerToReceiveLovelace } =
+      computeMarketplacePurchasePayouts(100_000_000, 250, "2000000");
+    expect(ownerToReceiveLovelace).toBe(2_500_000n);
+    expect(sellerToReceiveLovelace).toBe(102_000_000n);
+  });
+});
diff --git a/packages/mesh-contract/src/marketplace/purchase-fee-math.ts b/packages/mesh-contract/src/marketplace/purchase-fee-math.ts
new file mode 100644
index 000000000..66e8ed7aa
--- /dev/null
+++ b/packages/mesh-contract/src/marketplace/purchase-fee-math.ts
@@ -0,0 +1,24 @@
+/**
+ * BigInt-safe fee / payout math for MeshMarketplaceContract.purchaseAsset.
+ * On-chain datum ints may deserialize as bigint; mixing with Number throws
+ * TypeError: Cannot mix BigInt and other types.
+ */
+export function computeMarketplacePurchasePayouts(
+  priceInt: number | bigint,
+  feePercentageBasisPoint: number,
+  inputLovelace: string | number | bigint,
+): {
+  ownerToReceiveLovelace: bigint;
+  sellerToReceiveLovelace: bigint;
+} {
+  const price = BigInt(priceInt);
+  const feeBp = BigInt(feePercentageBasisPoint);
+  // Match prior Math.ceil(float) semantics for positive amounts.
+  let ownerToReceiveLovelace =
+    feeBp === 0n ? 0n : (price * feeBp + 9999n) / 10000n;
+  if (feePercentageBasisPoint > 0 && ownerToReceiveLovelace < 1000000n) {
+    ownerToReceiveLovelace = 1000000n;
+  }
+  const sellerToReceiveLovelace = price + BigInt(inputLovelace);
+  return { ownerToReceiveLovelace, sellerToReceiveLovelace };
+}