diff --git a/src/encoding_binding.cc b/src/encoding_binding.cc
index 8a445088b54a..dadaed04e2fe 100644
--- a/src/encoding_binding.cc
+++ b/src/encoding_binding.cc
@@ -10,6 +10,7 @@
 
 #include <algorithm>
 #include <cstdint>
+#include <type_traits>
 
 namespace node {
 namespace encoding_binding {
@@ -88,7 +89,7 @@ constexpr bool isSurrogatePair(uint16_t lead, uint16_t trail) {
 
 constexpr size_t simpleUtfEncodingLength(uint16_t c) {
   if (c < 0x80) return 1;
-  if (c < 0x400) return 2;
+  if (c < 0x800) return 2;
   return 3;
 }
 
@@ -162,7 +163,13 @@ size_t findBestFit(const Char* data, size_t length, size_t bufferSize) {
   }
 
   while (pos < length && utf8Accumulated < bufferSize) {
-    size_t extra = simpleUtfEncodingLength(data[pos]);
+    // `char` is signed on some platforms/ABIs, so widening a byte >= 0x80
+    // straight to uint16_t would sign-extend it into a bogus code point.
+    // Go through the Char type's unsigned counterpart first (a no-op for
+    // char16_t, which is unsigned already) to get the right code unit.
+    using UnsignedChar = std::make_unsigned_t<Char>;
+    size_t extra = simpleUtfEncodingLength(
+        static_cast<uint16_t>(static_cast<UnsignedChar>(data[pos])));
     if (utf8Accumulated + extra > bufferSize) break;
     pos++;
     utf8Accumulated += extra;
diff --git a/test/parallel/test-whatwg-encoding-custom-textencoder-encodeinto.js b/test/parallel/test-whatwg-encoding-custom-textencoder-encodeinto.js
new file mode 100644
index 000000000000..5633d6310910
--- /dev/null
+++ b/test/parallel/test-whatwg-encoding-custom-textencoder-encodeinto.js
@@ -0,0 +1,39 @@
+'use strict';
+
+// This tests that TextEncoder.encodeInto() does not underestimate how many
+// bytes a code point needs when computing how much of the source string fits
+// into the destination.
+
+require('../common');
+const assert = require('assert');
+
+// Long enough to bypass the small-string fast path (kSmallStringThreshold = 32
+// in src/encoding_binding.cc) and exercise the chunked encoding logic.
+const encoder = new TextEncoder();
+
+{
+  // Code points in [0x80, 0x800) are 2 bytes in UTF-8; treating them as 3
+  // bytes causes encodeInto() to reject input that would actually fit.
+  const text = 'Ѐ'.repeat(33);
+  const result = encoder.encodeInto(text, new Uint8Array(2));
+  assert.strictEqual(result.read, 1);
+  assert.strictEqual(result.written, 2);
+}
+
+{
+  // A one-byte (Latin1) source string takes a different internal path than
+  // a two-byte (UTF-16) one. Bytes >= 0x80 must be treated as unsigned there
+  // too, or they get sign-extended into a bogus, oversized code point.
+  const text = 'é'.repeat(33);
+  const result = encoder.encodeInto(text, new Uint8Array(2));
+  assert.strictEqual(result.read, 1);
+  assert.strictEqual(result.written, 2);
+
+  // Appending a two-byte character forces the whole string to be stored as
+  // UTF-16 internally, which must not change how the Latin1-only prefix
+  // encodes.
+  const withTrailingChar = encoder.encodeInto(
+    text + '☺', new Uint8Array(2));
+  assert.strictEqual(withTrailingChar.read, result.read);
+  assert.strictEqual(withTrailingChar.written, result.written);
+}