diff --git a/source/ulid.ts b/source/ulid.ts
index a270724..7492783 100644
--- a/source/ulid.ts
+++ b/source/ulid.ts
@@ -1,6 +1,6 @@
 import crypto from "node:crypto";
 import { incrementBase32 } from "./crockford.js";
-import { ENCODING, ENCODING_LEN, RANDOM_LEN, TIME_LEN, TIME_MAX } from "./constants.js";
+import { ENCODING, ENCODING_LEN, RANDOM_LEN, TIME_LEN, TIME_MAX, ULID_REGEX } from "./constants.js";
 import { ULIDError, ULIDErrorCode } from "./error.js";
 import { PRNG, ULID, ULIDFactory } from "./types.js";
 import { randomChar } from "./utils.js";
@@ -133,14 +133,9 @@ function inWebWorker(): boolean {
  *   isValid(""); // false
  */
 export function isValid(id: string): boolean {
-    return (
-        typeof id === "string" &&
-        id.length === TIME_LEN + RANDOM_LEN &&
-        id
-            .toUpperCase()
-            .split("")
-            .every(char => ENCODING.indexOf(char) !== -1)
-    );
+    // ULID_REGEX also constrains the leading character to 0-7, so a string whose
+    // timestamp exceeds the 48-bit maximum (which decodeTime rejects) is invalid.
+    return typeof id === "string" && ULID_REGEX.test(id);
 }
 
 /**
diff --git a/test/node/ulid.spec.ts b/test/node/ulid.spec.ts
index 8896506..5d4df71 100644
--- a/test/node/ulid.spec.ts
+++ b/test/node/ulid.spec.ts
@@ -1,5 +1,5 @@
 import { describe, expect, it } from "vitest";
-import { decodeTime, encodeTime, monotonicFactory, ulid, ULIDFactory } from "../../";
+import { decodeTime, encodeTime, isValid, monotonicFactory, ulid, ULIDFactory } from "../../";
 
 const ULID_REXP = /^[0-7][0-9a-hjkmnp-tv-zA-HJKMNP-TV-Z]{25}$/;
 
@@ -85,3 +85,37 @@ describe("ulid", () => {
         expect(id).toMatch(ULID_REXP);
     });
 });
+
+describe("isValid", () => {
+    it("accepts a generated ULID, in either case", () => {
+        const id = ulid();
+        expect(isValid(id)).toBe(true);
+        expect(isValid(id.toLowerCase())).toBe(true);
+    });
+
+    it("rejects non-strings, the wrong length and out-of-alphabet characters", () => {
+        expect(isValid("")).toBe(false);
+        expect(isValid("0".repeat(25))).toBe(false);
+        expect(isValid("0I" + "0".repeat(24))).toBe(false); // I is not in Crockford base32
+    });
+
+    it("rejects a timestamp larger than the 48-bit maximum", () => {
+        // The largest valid ULID is 7ZZ…; a leading character above 7 overflows
+        // the 48-bit timestamp, which decodeTime rejects — so isValid must too.
+        expect(isValid("8" + "0".repeat(25))).toBe(false);
+        expect(isValid("Z".repeat(26))).toBe(false);
+    });
+
+    it("agrees with decodeTime", () => {
+        const ids = ["7" + "Z".repeat(25), "8" + "0".repeat(25), "Z".repeat(26)];
+        for (const id of ids) {
+            let decodes = true;
+            try {
+                decodeTime(id);
+            } catch {
+                decodes = false;
+            }
+            expect(isValid(id)).toBe(decodes);
+        }
+    });
+});