diff --git a/CLAUDE.md b/CLAUDE.md
index 10b866d..48f18c8 100644
--- a/CLAUDE.md
+++ b/CLAUDE.md
@@ -48,6 +48,7 @@ src/
       update.ts                   - Update loan by ID
       pay.ts                      - Make a loan payment
       mark-paid.ts                - Pay the next pending installment
+      unmark-paid.ts              - Reverse a loan payment (default: most recent)
       delete.ts                   - Delete loan by ID
     stats/
       summary.ts                  - Financial summary
@@ -161,6 +162,7 @@ Login flow writes human-readable output to stderr (not JSON) since it is interac
 | GET    | /api/loans                       | loans list              |
 | POST   | /api/loans                       | loans create            |
 | POST   | /api/loans/:id/pay               | loans pay               |
+| POST   | /api/loans/:id/reverse-payment   | loans unmark-paid       |
 | PUT    | /api/loans/:id                   | loans update            |
 | DELETE | /api/loans/:id                   | loans delete            |
 | GET    | /api/stats/summary               | stats summary           |
diff --git a/src/commands/loans/unmark-paid.ts b/src/commands/loans/unmark-paid.ts
new file mode 100644
index 0000000..eb4d6b6
--- /dev/null
+++ b/src/commands/loans/unmark-paid.ts
@@ -0,0 +1,44 @@
+import { Command } from "commander";
+import { apiRequest } from "../../lib/api-client.js";
+import { output } from "../../lib/output.js";
+
+interface LoanPayment {
+  id: string;
+  paidAt: string;
+  payAmount: number;
+  loanAmount: number;
+  payCurrency: string;
+}
+
+interface LoanDetails {
+  id: string;
+  payments: LoanPayment[];
+}
+
+export const unmarkPaidLoanCommand = new Command("unmark-paid")
+  .description("Reverse a loan payment (default: most recent)")
+  .argument("<id>", "Loan ID")
+  .option("--payment-id <id>", "Specific payment ID to reverse")
+  .addHelpText(
+    "after",
+    "\nExamples:\n  lucas loans unmark-paid <loan-id>\n  lucas loans unmark-paid <loan-id> --payment-id <payment-id>\n",
+  )
+  .action(async (id: string, opts: { paymentId?: string }) => {
+    let paymentId = opts.paymentId;
+
+    if (!paymentId) {
+      const loan = await apiRequest<LoanDetails>("GET", `/api/loans/${id}`);
+      if (!loan.payments || loan.payments.length === 0) {
+        output.error("No payments found for this loan", 404, { loanId: id });
+      }
+      const sorted = [...loan.payments].sort(
+        (a, b) => new Date(b.paidAt).getTime() - new Date(a.paidAt).getTime(),
+      );
+      paymentId = sorted[0].id;
+    }
+
+    const data = await apiRequest("POST", `/api/loans/${id}/reverse-payment`, {
+      paymentId,
+    });
+    output.success(data);
+  });
diff --git a/src/index.ts b/src/index.ts
index 2d7242e..1b73a38 100644
--- a/src/index.ts
+++ b/src/index.ts
@@ -39,6 +39,7 @@ import { createLoanCommand } from "./commands/loans/create.js";
 import { updateLoanCommand } from "./commands/loans/update.js";
 import { payLoanCommand } from "./commands/loans/pay.js";
 import { markPaidLoanCommand } from "./commands/loans/mark-paid.js";
+import { unmarkPaidLoanCommand } from "./commands/loans/unmark-paid.js";
 import { deleteLoanCommand } from "./commands/loans/delete.js";
 
 // Stats
@@ -107,6 +108,7 @@ loans.addCommand(createLoanCommand);
 loans.addCommand(updateLoanCommand);
 loans.addCommand(payLoanCommand);
 loans.addCommand(markPaidLoanCommand);
+loans.addCommand(unmarkPaidLoanCommand);
 loans.addCommand(deleteLoanCommand);
 
 // Grupo: stats