diff --git a/src/lib/purchase-orders.ts b/src/lib/purchase-orders.ts
new file mode 100644
index 0000000..45c8970
--- /dev/null
+++ b/src/lib/purchase-orders.ts
@@ -0,0 +1,161 @@
+import { z } from "zod";
+import { Prisma } from "@prisma/client";
+import { prisma } from "./prisma";
+
+export interface PurchaseOrderRecord {
+  id: string;
+  vendor_id: string;
+  order_date: string;
+  expected_date: string;
+  received_at: string | null;
+  received_by: string | null;
+  created_at: string;
+}
+
+export interface CreatePurchaseOrderInput {
+  vendorId: string;
+  orderDate: Date;
+  expectedDate: Date;
+  receivedAt?: Date | null;
+  receivedById?: string | null;
+}
+
+export type ValidationResult =
+  | { ok: true; value: CreatePurchaseOrderInput }
+  | { ok: false; error: string };
+
+// Mirrors src/lib/licenses.ts's dateString helper — accepts any string
+// Date.parse() can handle and transforms it into a Date for Prisma. Used
+// here for orderDate/expectedDate/receivedAt, which are all Timestamptz
+// (not just Date) columns, so datetime strings (not just "2026-07-31") are
+// also accepted.
+function dateString(message: string) {
+  return z
+    .string({ required_error: message, invalid_type_error: message })
+    .refine((value) => !Number.isNaN(Date.parse(value)), { message })
+    .transform((value) => new Date(value));
+}
+
+const createPurchaseOrderSchema = z
+  .object({
+    // FK to Vendor — only a UUID *format* check here, whether it references
+    // a real vendor is left to Prisma's own FK constraint at insert time
+    // (see createPurchaseOrder()'s P2003 handling below). Mirrors
+    // src/lib/licenses.ts's vendorId handling.
+    vendorId: z
+      .string({ required_error: "vendorId is required and must be a valid UUID string" })
+      .uuid("vendorId is required and must be a valid UUID string"),
+    orderDate: dateString("orderDate is required and must be a valid date string"),
+    expectedDate: dateString("expectedDate is required and must be a valid date string"),
+    // Both nullable — null means "not yet received". Co-nullability with
+    // receivedById is enforced by the .refine() below, not here.
+    receivedAt: dateString("receivedAt must be a valid date string").optional().nullable(),
+    receivedById: z
+      .string({ invalid_type_error: "receivedById must be a valid UUID string" })
+      .uuid("receivedById must be a valid UUID string")
+      .optional()
+      .nullable(),
+  })
+  .refine(
+    (value) => {
+      const hasReceivedAt = value.receivedAt !== undefined && value.receivedAt !== null;
+      const hasReceivedById = value.receivedById !== undefined && value.receivedById !== null;
+      return hasReceivedAt === hasReceivedById;
+    },
+    {
+      message: "receivedAt and receivedById must both be set or both be unset",
+      // Attach the error to both fields so it surfaces regardless of which
+      // one the caller looks at first.
+      path: ["receivedAt"],
+    }
+  );
+
+/**
+ * Validates raw (untyped) purchase-order-creation input. vendorId must be a
+ * valid UUID string, orderDate/expectedDate must be valid date strings,
+ * receivedAt/receivedById are optional/nullable but must both be set or
+ * both be unset (co-nullability invariant — receiving a PO always records
+ * both "when" and "who" together). No status field is accepted — PurchaseOrder
+ * deliberately has no stored status (see schema.prisma comment), it's
+ * inferred from linked Requests.
+ */
+export function validateCreatePurchaseOrderInput(input: {
+  vendorId?: unknown;
+  orderDate?: unknown;
+  expectedDate?: unknown;
+  receivedAt?: unknown;
+  receivedById?: unknown;
+}): ValidationResult {
+  const result = createPurchaseOrderSchema.safeParse(input);
+  if (!result.success) {
+    const message = result.error.issues[0]?.message ?? "Invalid input";
+    return { ok: false, error: message };
+  }
+  return { ok: true, value: result.data };
+}
+
+function toRecord(row: {
+  id: string;
+  vendorId: string;
+  orderDate: Date;
+  expectedDate: Date;
+  receivedAt: Date | null;
+  receivedById: string | null;
+  createdAt: Date;
+}): PurchaseOrderRecord {
+  return {
+    id: row.id,
+    vendor_id: row.vendorId,
+    order_date: row.orderDate.toISOString(),
+    expected_date: row.expectedDate.toISOString(),
+    received_at: row.receivedAt ? row.receivedAt.toISOString() : null,
+    received_by: row.receivedById,
+    created_at: row.createdAt.toISOString(),
+  };
+}
+
+export type CreatePurchaseOrderResult =
+  | { ok: true; value: PurchaseOrderRecord }
+  | { ok: false; error: string };
+
+/**
+ * Creates a PurchaseOrder. Two separate FKs can fail here (vendorId ->
+ * Vendor, receivedById -> Employee) — Prisma raises the same error code
+ * (P2003) for either, so the FK constraint name in `error.meta.field_name`
+ * is used to tell them apart and return a targeted message, rather than a
+ * raw Prisma error leaking to the API layer. Mirrors
+ * src/lib/licenses.ts's createLicense() pattern, extended for the
+ * two-FK case.
+ */
+export async function createPurchaseOrder(
+  input: CreatePurchaseOrderInput
+): Promise<CreatePurchaseOrderResult> {
+  try {
+    const created = await prisma.purchaseOrder.create({
+      data: {
+        vendorId: input.vendorId,
+        orderDate: input.orderDate,
+        expectedDate: input.expectedDate,
+        receivedAt: input.receivedAt ?? null,
+        receivedById: input.receivedById ?? null,
+      },
+    });
+    return { ok: true, value: toRecord(created) };
+  } catch (error) {
+    if (error instanceof Prisma.PrismaClientKnownRequestError && error.code === "P2003") {
+      const fieldName = typeof error.meta?.field_name === "string" ? error.meta.field_name : "";
+      if (fieldName.includes("received_by")) {
+        return { ok: false, error: "receivedById does not reference an existing employee" };
+      }
+      return { ok: false, error: "vendorId does not reference an existing vendor" };
+    }
+    throw error;
+  }
+}
+
+export async function listPurchaseOrders(): Promise<PurchaseOrderRecord[]> {
+  const rows = await prisma.purchaseOrder.findMany({
+    orderBy: [{ createdAt: "desc" }, { id: "desc" }],
+  });
+  return rows.map(toRecord);
+}