diff --git a/ark/type/__tests__/keywords/string.test.ts b/ark/type/__tests__/keywords/string.test.ts
index 479d19b9a..03e56e14c 100644
--- a/ark/type/__tests__/keywords/string.test.ts
+++ b/ark/type/__tests__/keywords/string.test.ts
@@ -43,6 +43,17 @@ contextualize(() => {
 		attest(b64url("fn5+").toString()).equals(
 			'must be base64url-encoded (was "fn5+")'
 		)
+
+		const b64parse = type("string.base64.parse")
+		attest(Buffer.from(b64parse("fn5+") as Uint8Array).toString("utf8")).snap(
+			"~~~"
+		)
+		attest(
+			Buffer.from(b64parse("V29yZA==") as Uint8Array).toString("utf8")
+		).snap("Word")
+		attest(b64("V29yZA").toString()).equals(
+			'must be base64-encoded (was "V29yZA")'
+		)
 	})
 
 	it("digits", () => {
diff --git a/ark/type/keywords/string/base64.ts b/ark/type/keywords/string/base64.ts
index ed03af347..b21965c77 100644
--- a/ark/type/keywords/string/base64.ts
+++ b/ark/type/keywords/string/base64.ts
@@ -1,17 +1,117 @@
+import { rootSchema, type TraversalContext } from "@ark/schema"
 import type { Module, Submodule } from "../../module.ts"
-import type { Predicate, of } from "../inference.ts"
+import type { Predicate, To, of } from "../inference.ts"
 import { arkModule } from "../utils.ts"
 import { regexStringNode } from "./utils.ts"
 
+// Using JS-only solution to parse Base64 as `Buffer.from` isn't available in browsers and `btoa` is
+// notoriously slow.
+//
+// Code adapted from base64-js: https://github.com/feross/base64-js/blob/master/index.js
+
+const lookup =
+	"ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/".split("")
+const revLookup = lookup.reduce(
+	(obj, char, i) => {
+		obj[char.charCodeAt(0)] = i
+		return obj
+	},
+	{} as Record<number, number>
+)
+
+const getLens = (b64: string) => {
+	const len = b64.length
+
+	if (len % 4 > 0)
+		throw new SyntaxError("Invalid string. Length must be a multiple of 4")
+
+	// Trim off extra bytes after placeholder bytes are found
+	// See: https://github.com/beatgammit/base64-js/issues/42
+	let validLen = b64.indexOf("=")
+	if (validLen === -1) validLen = len
+
+	const placeHoldersLen = validLen === len ? 0 : 4 - (validLen % 4)
+
+	return [validLen, placeHoldersLen]
+}
+
+const byteLength = (validLen: number, placeHoldersLen: number) =>
+	((validLen + placeHoldersLen) * 3) / 4 - placeHoldersLen
+
+const parseB64 = (b64: string) => {
+	const [validLen, placeHoldersLen] = getLens(b64)
+	const arr = new Uint8Array(byteLength(validLen, placeHoldersLen))
+
+	// if there are placeholders, only get up to the last complete 4 chars
+	const len = placeHoldersLen > 0 ? validLen - 4 : validLen
+
+	let tmp: number
+	let curByte = 0
+
+	let i: number
+	for (i = 0; i < len; i += 4) {
+		tmp =
+			(revLookup[b64.charCodeAt(i)] << 18) |
+			(revLookup[b64.charCodeAt(i + 1)] << 12) |
+			(revLookup[b64.charCodeAt(i + 2)] << 6) |
+			revLookup[b64.charCodeAt(i + 3)]
+		arr[curByte++] = (tmp >> 16) & 0xff
+		arr[curByte++] = (tmp >> 8) & 0xff
+		arr[curByte++] = tmp & 0xff
+	}
+
+	if (placeHoldersLen === 2) {
+		tmp =
+			(revLookup[b64.charCodeAt(i)] << 2) |
+			(revLookup[b64.charCodeAt(i + 1)] >> 4)
+		arr[curByte++] = tmp & 0xff
+	}
+
+	if (placeHoldersLen === 1) {
+		tmp =
+			(revLookup[b64.charCodeAt(i)] << 10) |
+			(revLookup[b64.charCodeAt(i + 1)] << 4) |
+			(revLookup[b64.charCodeAt(i + 2)] >> 2)
+		arr[curByte++] = (tmp >> 8) & 0xff
+		arr[curByte++] = tmp & 0xff
+	}
+
+	return arr
+}
+
+const base64Description = "base64-encoded"
+const base64UrlDescription = "base64url-encoded"
+
+export const writeBase64SyntaxErrorProblem = (error: unknown): string => {
+	if (!(error instanceof SyntaxError)) throw error
+	return `must be ${base64Description} (${error})`
+}
+
+const base64Pattern =
+	/^(?:[A-Za-z0-9+/]{4})*(?:[A-Za-z0-9+/]{2}==|[A-Za-z0-9+/]{3}=)?$/
+const base64UrlPattern =
+	/^(?:[A-Za-z0-9_-]{4})*(?:[A-Za-z0-9_-]{2}(?:==|%3D%3D)?|[A-Za-z0-9_-]{3}(?:=|%3D)?)?$/
+
 export const base64 = arkModule({
-	root: regexStringNode(
-		/^(?:[A-Za-z0-9+/]{4})*(?:[A-Za-z0-9+/]{2}==|[A-Za-z0-9+/]{3}=)?$/,
-		"base64-encoded"
-	),
-	url: regexStringNode(
-		/^(?:[A-Za-z0-9_-]{4})*(?:[A-Za-z0-9_-]{2}(?:==|%3D%3D)?|[A-Za-z0-9_-]{3}(?:=|%3D)?)?$/,
-		"base64url-encoded"
-	)
+	root: regexStringNode(base64Pattern, base64Description),
+	url: regexStringNode(base64UrlPattern, base64UrlDescription),
+	parse: rootSchema({
+		in: "string",
+		declaredOut: rootSchema(Uint8Array),
+		morphs: (s: string, ctx: TraversalContext) => {
+			if (s.length === 0) return new Uint8Array(0)
+
+			try {
+				return parseB64(s)
+			} catch (e) {
+				return ctx.error({
+					code: "predicate",
+					expected: base64Description,
+					problem: writeBase64SyntaxErrorProblem(e)
+				})
+			}
+		}
+	})
 })
 
 declare namespace string {
@@ -19,6 +119,7 @@ declare namespace string {
 
 	export namespace base64 {
 		export type url = of<string, Predicate<"base64.url">>
+		export type parse = of<string, Predicate<"base64.parse">>
 	}
 }
 
@@ -30,5 +131,6 @@ export declare namespace base64 {
 	export type $ = {
 		root: string.base64
 		url: string.base64.url
+		parse: (In: string.base64) => To<Uint8Array>
 	}
 }