diff --git a/src/json/index.test.ts b/src/json/index.test.ts
index 825bbc0e7..d37a86c37 100644
--- a/src/json/index.test.ts
+++ b/src/json/index.test.ts
@@ -44,3 +44,26 @@ test("validateSchema - optional properties are optional", async (t) => {
   t.true(json.validateSchema(optionalSchema, { optionalKey: "" }));
   t.true(json.validateSchema(optionalSchema, { optionalKey: "foo" }));
 });
+
+const undefinableSchema = {
+  optionalKey: json.undefinable(json.string),
+};
+
+test("validateSchema - undefinable properties are optional but reject null", async (t) => {
+  // Optional fields may be absent or explicitly undefined
+  t.true(json.validateSchema(undefinableSchema, {}));
+  t.true(json.validateSchema(undefinableSchema, { optionalKey: undefined }));
+
+  // But, unlike `optional`, `null` is rejected
+  t.false(json.validateSchema(undefinableSchema, { optionalKey: null }));
+
+  // And, if present, should have the expected type
+  t.false(json.validateSchema(undefinableSchema, { optionalKey: 0 }));
+  t.false(json.validateSchema(undefinableSchema, { optionalKey: 123 }));
+  t.false(json.validateSchema(undefinableSchema, { optionalKey: false }));
+  t.false(json.validateSchema(undefinableSchema, { optionalKey: true }));
+  t.false(json.validateSchema(undefinableSchema, { optionalKey: [] }));
+  t.false(json.validateSchema(undefinableSchema, { optionalKey: {} }));
+  t.true(json.validateSchema(undefinableSchema, { optionalKey: "" }));
+  t.true(json.validateSchema(undefinableSchema, { optionalKey: "foo" }));
+});
diff --git a/src/json/index.ts b/src/json/index.ts
index 8a1b60a17..4a3f65947 100644
--- a/src/json/index.ts
+++ b/src/json/index.ts
@@ -30,6 +30,11 @@ export function isString(value: unknown): value is string {
   return typeof value === "string";
 }
 
+/** Asserts that `value` is a number. */
+export function isNumber(value: unknown): value is number {
+  return typeof value === "number";
+}
+
 /** Asserts that `value` is either a string or undefined. */
 export function isStringOrUndefined(
   value: unknown,
@@ -55,7 +60,22 @@ export const string = {
   required: true,
 } as const satisfies Validator<string>;
 
-/** Transforms a validator to be optional. */
+/** A validator for number fields in schemas. */
+export const number = {
+  validate: isNumber,
+  required: true,
+} as const satisfies Validator<number>;
+
+/** A validator for object fields in schemas. */
+export const object = {
+  validate: isObject,
+  required: true,
+} as const satisfies Validator<UnvalidatedObject<unknown>>;
+
+/**
+ * Transforms a validator to be optional, accepting `undefined` or `null` for an
+ * absent value.
+ */
 export function optional<T>(validator: Validator<T>) {
   return {
     validate: (val: unknown) => {
@@ -65,6 +85,19 @@ export function optional<T>(validator: Validator<T>) {
   } as const satisfies Validator<T | undefined | null>;
 }
 
+/**
+ * Transforms a validator to be optional, accepting `undefined` for an absent
+ * value but, unlike `optional`, rejecting `null`.
+ */
+export function undefinable<T>(validator: Validator<T>) {
+  return {
+    validate: (val: unknown): val is T | undefined => {
+      return val === undefined || validator.validate(val);
+    },
+    required: false,
+  } as const satisfies Validator<T | undefined>;
+}
+
 /** Represents an arbitrary object schema. */
 export type Schema = Record<string, Validator<any>>;