diff --git a/internal/http3/qpack.go b/internal/http3/qpack.go
new file mode 100644
index 000000000..5dcdf6a6a
--- /dev/null
+++ b/internal/http3/qpack.go
@@ -0,0 +1,153 @@
+// Copyright 2025 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+//go:build go1.24
+
+package http3
+
+import (
+	"io"
+
+	"golang.org/x/net/http2/hpack"
+)
+
+// Prefixed-integer encoding from RFC 7541, section 5.1
+//
+// Prefixed integers consist of some number of bits of data,
+// N bits of encoded integer, and 0 or more additional bytes of
+// encoded integer.
+//
+// The RFCs represent this as, for example:
+//
+//       0   1   2   3   4   5   6   7
+//     +---+---+---+---+---+---+---+---+
+//     | 0 | 0 | 1 |   Capacity (5+)   |
+//     +---+---+---+-------------------+
+//
+// "Capacity" is an integer with a 5-bit prefix.
+//
+// In the following functions, a "prefixLen" parameter is the number
+// of integer bits in the first byte (5 in the above example), and
+// a "firstByte" parameter is a byte containing the first byte of
+// the encoded value (0x001x_xxxx in the above example).
+//
+// https://www.rfc-editor.org/rfc/rfc9204.html#section-4.1.1
+// https://www.rfc-editor.org/rfc/rfc7541#section-5.1
+
+// readPrefixedInt reads an RFC 7541 prefixed integer from st.
+func (st *stream) readPrefixedInt(prefixLen uint8) (firstByte byte, v int64, err error) {
+	firstByte, err = st.ReadByte()
+	if err != nil {
+		return 0, 0, errQPACKDecompressionFailed
+	}
+	v, err = st.readPrefixedIntWithByte(firstByte, prefixLen)
+	return firstByte, v, err
+}
+
+// readPrefixedInt reads an RFC 7541 prefixed integer from st.
+// The first byte has already been read from the stream.
+func (st *stream) readPrefixedIntWithByte(firstByte byte, prefixLen uint8) (v int64, err error) {
+	prefixMask := (byte(1) << prefixLen) - 1
+	v = int64(firstByte & prefixMask)
+	if v != int64(prefixMask) {
+		return v, nil
+	}
+	m := 0
+	for {
+		b, err := st.ReadByte()
+		if err != nil {
+			return 0, errQPACKDecompressionFailed
+		}
+		v += int64(b&127) << m
+		m += 7
+		if b&128 == 0 {
+			break
+		}
+	}
+	return v, err
+}
+
+// appendPrefixedInt appends an RFC 7541 prefixed integer to b.
+//
+// The firstByte parameter includes the non-integer bits of the first byte.
+// The other bits must be zero.
+func appendPrefixedInt(b []byte, firstByte byte, prefixLen uint8, i int64) []byte {
+	u := uint64(i)
+	prefixMask := (uint64(1) << prefixLen) - 1
+	if u < prefixMask {
+		return append(b, firstByte|byte(u))
+	}
+	b = append(b, firstByte|byte(prefixMask))
+	u -= prefixMask
+	for u >= 128 {
+		b = append(b, 0x80|byte(u&0x7f))
+		u >>= 7
+	}
+	return append(b, byte(u))
+}
+
+// String literal encoding from RFC 7541, section 5.2
+//
+// String literals consist of a single bit flag indicating
+// whether the string is Huffman-encoded, a prefixed integer (see above),
+// and the string.
+//
+// https://www.rfc-editor.org/rfc/rfc9204.html#section-4.1.2
+// https://www.rfc-editor.org/rfc/rfc7541#section-5.2
+
+// readPrefixedString reads an RFC 7541 string from st.
+func (st *stream) readPrefixedString(prefixLen uint8) (firstByte byte, s string, err error) {
+	firstByte, err = st.ReadByte()
+	if err != nil {
+		return 0, "", errQPACKDecompressionFailed
+	}
+	s, err = st.readPrefixedStringWithByte(firstByte, prefixLen)
+	return firstByte, s, err
+}
+
+// readPrefixedString reads an RFC 7541 string from st.
+// The first byte has already been read from the stream.
+func (st *stream) readPrefixedStringWithByte(firstByte byte, prefixLen uint8) (s string, err error) {
+	size, err := st.readPrefixedIntWithByte(firstByte, prefixLen)
+	if err != nil {
+		return "", errQPACKDecompressionFailed
+	}
+
+	hbit := byte(1) << prefixLen
+	isHuffman := firstByte&hbit != 0
+
+	// TODO: Avoid allocating here.
+	data := make([]byte, size)
+	if _, err := io.ReadFull(st, data); err != nil {
+		return "", errQPACKDecompressionFailed
+	}
+	if isHuffman {
+		// TODO: Move Huffman functions into a new package that hpack (HTTP/2)
+		// and this package can both import. Most of the hpack package isn't
+		// relevant to HTTP/3.
+		s, err := hpack.HuffmanDecodeToString(data)
+		if err != nil {
+			return "", errQPACKDecompressionFailed
+		}
+		return s, nil
+	}
+	return string(data), nil
+}
+
+// appendPrefixedString appends an RFC 7541 string to st.
+//
+// The firstByte parameter includes the non-integer bits of the first byte.
+// The other bits must be zero.
+func appendPrefixedString(b []byte, firstByte byte, prefixLen uint8, s string) []byte {
+	huffmanLen := hpack.HuffmanEncodeLength(s)
+	if huffmanLen < uint64(len(s)) {
+		hbit := byte(1) << prefixLen
+		b = appendPrefixedInt(b, firstByte|hbit, prefixLen, int64(huffmanLen))
+		b = hpack.AppendHuffmanString(b, s)
+	} else {
+		b = appendPrefixedInt(b, firstByte, prefixLen, int64(len(s)))
+		b = append(b, s...)
+	}
+	return b
+}
diff --git a/internal/http3/qpack_test.go b/internal/http3/qpack_test.go
new file mode 100644
index 000000000..6e16511fc
--- /dev/null
+++ b/internal/http3/qpack_test.go
@@ -0,0 +1,173 @@
+// Copyright 2025 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+//go:build go1.24
+
+package http3
+
+import (
+	"bytes"
+	"testing"
+)
+
+func TestPrefixedInt(t *testing.T) {
+	st1, st2 := newStreamPair(t)
+	for _, test := range []struct {
+		value     int64
+		prefixLen uint8
+		encoded   []byte
+	}{
+		// https://www.rfc-editor.org/rfc/rfc7541#appendix-C.1.1
+		{
+			value:     10,
+			prefixLen: 5,
+			encoded: []byte{
+				0b_0000_1010,
+			},
+		},
+		// https://www.rfc-editor.org/rfc/rfc7541#appendix-C.1.2
+		{
+			value:     1337,
+			prefixLen: 5,
+			encoded: []byte{
+				0b0001_1111,
+				0b1001_1010,
+				0b0000_1010,
+			},
+		},
+		// https://www.rfc-editor.org/rfc/rfc7541#appendix-C.1.3
+		{
+			value:     42,
+			prefixLen: 8,
+			encoded: []byte{
+				0b0010_1010,
+			},
+		},
+	} {
+		highBitMask := ^((byte(1) << test.prefixLen) - 1)
+		for _, highBits := range []byte{
+			0, highBitMask, 0b1010_1010 & highBitMask,
+		} {
+			gotEnc := appendPrefixedInt(nil, highBits, test.prefixLen, test.value)
+			wantEnc := append([]byte{}, test.encoded...)
+			wantEnc[0] |= highBits
+			if !bytes.Equal(gotEnc, wantEnc) {
+				t.Errorf("appendPrefixedInt(nil, 0b%08b, %v, %v) = {%x}, want {%x}",
+					highBits, test.prefixLen, test.value, gotEnc, wantEnc)
+			}
+
+			st1.Write(gotEnc)
+			if err := st1.Flush(); err != nil {
+				t.Fatal(err)
+			}
+			gotFirstByte, v, err := st2.readPrefixedInt(test.prefixLen)
+			if err != nil || gotFirstByte&highBitMask != highBits || v != test.value {
+				t.Errorf("st.readPrefixedInt(%v) = 0b%08b, %v, %v; want 0b%08b, %v, nil", test.prefixLen, gotFirstByte, v, err, highBits, test.value)
+			}
+		}
+	}
+}
+
+func TestPrefixedString(t *testing.T) {
+	st1, st2 := newStreamPair(t)
+	for _, test := range []struct {
+		value     string
+		prefixLen uint8
+		encoded   []byte
+	}{
+		// https://www.rfc-editor.org/rfc/rfc7541#appendix-C.6.1
+		{
+			value:     "302",
+			prefixLen: 7,
+			encoded: []byte{
+				0x82, // H bit + length 2
+				0x64, 0x02,
+			},
+		},
+		{
+			value:     "private",
+			prefixLen: 5,
+			encoded: []byte{
+				0x25, // H bit + length 5
+				0xae, 0xc3, 0x77, 0x1a, 0x4b,
+			},
+		},
+		{
+			value:     "Mon, 21 Oct 2013 20:13:21 GMT",
+			prefixLen: 7,
+			encoded: []byte{
+				0x96, // H bit + length 22
+				0xd0, 0x7a, 0xbe, 0x94, 0x10, 0x54, 0xd4, 0x44,
+				0xa8, 0x20, 0x05, 0x95, 0x04, 0x0b, 0x81, 0x66,
+				0xe0, 0x82, 0xa6, 0x2d, 0x1b, 0xff,
+			},
+		},
+		{
+			value:     "https://www.example.com",
+			prefixLen: 7,
+			encoded: []byte{
+				0x91, // H bit + length 17
+				0x9d, 0x29, 0xad, 0x17, 0x18, 0x63, 0xc7, 0x8f,
+				0x0b, 0x97, 0xc8, 0xe9, 0xae, 0x82, 0xae, 0x43,
+				0xd3,
+			},
+		},
+		// Not Huffman encoded (encoded size == unencoded size).
+		{
+			value:     "a",
+			prefixLen: 7,
+			encoded: []byte{
+				0x01, // length 1
+				0x61,
+			},
+		},
+		// Empty string.
+		{
+			value:     "",
+			prefixLen: 7,
+			encoded: []byte{
+				0x00, // length 0
+			},
+		},
+	} {
+		highBitMask := ^((byte(1) << (test.prefixLen + 1)) - 1)
+		for _, highBits := range []byte{
+			0, highBitMask, 0b1010_1010 & highBitMask,
+		} {
+			gotEnc := appendPrefixedString(nil, highBits, test.prefixLen, test.value)
+			wantEnc := append([]byte{}, test.encoded...)
+			wantEnc[0] |= highBits
+			if !bytes.Equal(gotEnc, wantEnc) {
+				t.Errorf("appendPrefixedString(nil, 0b%08b, %v, %v) = {%x}, want {%x}",
+					highBits, test.prefixLen, test.value, gotEnc, wantEnc)
+			}
+
+			st1.Write(gotEnc)
+			if err := st1.Flush(); err != nil {
+				t.Fatal(err)
+			}
+			gotFirstByte, v, err := st2.readPrefixedString(test.prefixLen)
+			if err != nil || gotFirstByte&highBitMask != highBits || v != test.value {
+				t.Errorf("st.readPrefixedInt(%v) = 0b%08b, %q, %v; want 0b%08b, %q, nil", test.prefixLen, gotFirstByte, v, err, highBits, test.value)
+			}
+		}
+	}
+}
+
+func TestHuffmanDecodingFailure(t *testing.T) {
+	st1, st2 := newStreamPair(t)
+	st1.Write([]byte{
+		0x82, // H bit + length 4
+		0b_1111_1111,
+		0b_1111_1111,
+		0b_1111_1111,
+		0b_1111_1111,
+	})
+	if err := st1.Flush(); err != nil {
+		t.Fatal(err)
+	}
+	if b, v, err := st2.readPrefixedString(7); err == nil {
+		t.Fatalf("readPrefixedString(7) = %x, %v, nil; want error", b, v)
+	}
+}