diff --git a/test/built-ins/Uint8Array/fromBase64/last-chunk-invalid.js b/test/built-ins/Uint8Array/fromBase64/last-chunk-invalid.js
new file mode 100644
index 0000000000..14056a9a50
--- /dev/null
+++ b/test/built-ins/Uint8Array/fromBase64/last-chunk-invalid.js
@@ -0,0 +1,90 @@
+// Copyright (C) 2025 Nikita Skovoroda. All rights reserved.
+// This code is governed by the BSD license found in the LICENSE file.
+/*---
+esid: sec-uint8array.frombase64
+description: Uint8Array.fromBase64 throws a SyntaxError when chunk size is invalid or padding is invalid
+features: [uint8array-base64, TypedArray]
+---*/
+
+// Non-padded incomplete chunk 'A'
+assert.throws(SyntaxError, function() {
+  Uint8Array.fromBase64('A');
+});
+assert.throws(SyntaxError, function() {
+  Uint8Array.fromBase64('A', { lastChunkHandling: 'loose' });
+});
+assert.throws(SyntaxError, function() {
+  Uint8Array.fromBase64('A', { lastChunkHandling: 'strict' });
+});
+assert.compareArray(Uint8Array.fromBase64('A', { lastChunkHandling: 'stop-before-partial' }), []);
+
+// Non-padded incomplete chunk 'ABCDA'
+assert.throws(SyntaxError, function() {
+  Uint8Array.fromBase64('ABCDA');
+});
+assert.throws(SyntaxError, function() {
+  Uint8Array.fromBase64('ABCDA', { lastChunkHandling: 'loose' });
+});
+assert.throws(SyntaxError, function() {
+  Uint8Array.fromBase64('ABCDA', { lastChunkHandling: 'strict' });
+});
+assert.compareArray(Uint8Array.fromBase64('ABCDA', { lastChunkHandling: 'stop-before-partial' }), [0, 16, 131]);
+
+// Incomplete padding in chunk 'AA=' is allowed but skipped in 'stop-before-partial', but not other modes
+assert.throws(SyntaxError, function() {
+  Uint8Array.fromBase64('AA=');
+});
+assert.throws(SyntaxError, function() {
+  Uint8Array.fromBase64('AA=', { lastChunkHandling: 'loose' });
+});
+assert.throws(SyntaxError, function() {
+  Uint8Array.fromBase64('AA=', { lastChunkHandling: 'strict' });
+});
+assert.compareArray(Uint8Array.fromBase64('AA=', { lastChunkHandling: 'stop-before-partial' }), []);
+assert.compareArray(Uint8Array.fromBase64('aQ=', { lastChunkHandling: 'stop-before-partial' }), []);
+assert.compareArray(Uint8Array.fromBase64('ABCDAA=', { lastChunkHandling: 'stop-before-partial' }), [0, 16, 131]);
+
+// Padded chunks always throw when incomplete before padding
+var illegal = [
+  '=',
+  '==',
+  '===',
+  '====',
+  '=====',
+  'A=',
+  'A==',
+  'A===',
+  'A====',
+  'A=====',
+  'AA====',
+  'AA=====',
+  'AAA==',
+  'AAA===',
+  'AAA====',
+  'AAA=====',
+  'AAAA=',
+  'AAAA==',
+  'AAAA===',
+  'AAAA====',
+  'AAAA=====',
+  'AAAAA=',
+  'AAAAA==',
+  'AAAAA===',
+  'AAAAA====',
+  'AAAAA=====',
+];
+
+illegal.forEach(function(value) {
+  assert.throws(SyntaxError, function() {
+    Uint8Array.fromBase64(value);
+  });
+  assert.throws(SyntaxError, function() {
+    Uint8Array.fromBase64(value, { lastChunkHandling: 'loose' });
+  });
+  assert.throws(SyntaxError, function() {
+    Uint8Array.fromBase64(value, { lastChunkHandling: 'strict' });
+  });
+  assert.throws(SyntaxError, function() {
+    Uint8Array.fromBase64(value, { lastChunkHandling: 'stop-before-partial' });
+  });
+});