diff --git a/src/declaration/network-port-number.ts b/src/declaration/network-port-number.ts
index 21dd3a5..83900a9 100644
--- a/src/declaration/network-port-number.ts
+++ b/src/declaration/network-port-number.ts
@@ -1,3 +1,7 @@
+import type {
+  Constraint,
+  DeclarationConstraintOptions,
+} from "../constraint.js";
 import { createNetworkPortNumberConstraint } from "../constraint/network-port-number.js";
 import {
   assertRangeSpec,
@@ -23,6 +27,7 @@ import { resolve } from "../maybe.js";
 import { ScalarSchema, createScalar, toString } from "../schema.js";
 
 export type Options = DeclarationOptions<number> &
+  DeclarationConstraintOptions<number> &
   DeclarationExampleOptions<number> &
   Partial<RangeConstraintSpec<number>>;
 
@@ -62,7 +67,10 @@ function createSchema(name: string, options: Options): ScalarSchema<number> {
     return Number(v);
   }
 
-  const constraints = [createNetworkPortNumberConstraint()];
+  const { constraints: explicitConstraints = [] } = options;
+  const constraints: Constraint<number>[] = [
+    createNetworkPortNumberConstraint(),
+  ];
 
   try {
     if (hasNumberRangeConstraint(options)) {
@@ -73,7 +81,10 @@ function createSchema(name: string, options: Options): ScalarSchema<number> {
     throw new SpecError(name, normalize(error));
   }
 
-  return createScalar("port number", toString, unmarshal, constraints);
+  return createScalar("port number", toString, unmarshal, [
+    ...constraints,
+    ...explicitConstraints,
+  ]);
 }
 
 function buildExamples(): Example<number>[] {
diff --git a/test/suite/declaration/network-port-number.spec.ts b/test/suite/declaration/network-port-number.spec.ts
index d9b6962..fd64087 100644
--- a/test/suite/declaration/network-port-number.spec.ts
+++ b/test/suite/declaration/network-port-number.spec.ts
@@ -298,4 +298,72 @@ describe("Network port number declarations", () => {
       });
     },
   );
+
+  describe("when the declaration has constraints", () => {
+    beforeEach(() => {
+      declaration = networkPortNumber("AUSTENITE_PORT", "<description>", {
+        constraints: [
+          {
+            description: "<constraint A>",
+            constrain: (v) => v % 2 === 0 || "must be divisible by 2",
+          },
+          {
+            description: "<constraint B>",
+            constrain: (v) => v % 3 === 0 || "must be divisible by 3",
+          },
+        ],
+        examples: [{ value: 6, label: "example" }],
+      });
+    });
+
+    describe("when the value satisfies the constraints", () => {
+      beforeEach(() => {
+        process.env.AUSTENITE_PORT = "6";
+
+        initialize({ onInvalid: noop });
+      });
+
+      describe(".value()", () => {
+        it("returns the value", () => {
+          expect(declaration.value()).toBe(6);
+        });
+      });
+    });
+
+    describe("when the value violates the first constraint", () => {
+      beforeEach(() => {
+        process.env.AUSTENITE_PORT = "3";
+
+        initialize({ onInvalid: noop });
+      });
+
+      describe(".value()", () => {
+        it("throws", () => {
+          expect(() => {
+            declaration.value();
+          }).toThrow(
+            "value of AUSTENITE_PORT (3) is invalid: must be divisible by 2",
+          );
+        });
+      });
+    });
+
+    describe("when the value violates the second constraint", () => {
+      beforeEach(() => {
+        process.env.AUSTENITE_PORT = "2";
+
+        initialize({ onInvalid: noop });
+      });
+
+      describe(".value()", () => {
+        it("throws", () => {
+          expect(() => {
+            declaration.value();
+          }).toThrow(
+            "value of AUSTENITE_PORT (2) is invalid: must be divisible by 3",
+          );
+        });
+      });
+    });
+  });
 });