diff --git a/json/hex.go b/json/hex.go
new file mode 100644
index 0000000..0845454
--- /dev/null
+++ b/json/hex.go
@@ -0,0 +1,62 @@
+package json
+
+import (
+	"encoding/hex"
+	"errors"
+	"fmt"
+)
+
+type HexBytes []byte
+
+func (b HexBytes) MarshalJSON() ([]byte, error) {
+	if b == nil {
+		return []byte("null"), nil
+	}
+
+	// Two bytes for the quotes, two bytes for the 0x prefix.
+	outLen := 4 + hex.EncodedLen(len(b))
+
+	out := make([]byte, outLen)
+	copy(out, `"0x`)
+	out[outLen-1] = '"'
+	hex.Encode(out[3:], b)
+	return out, nil
+}
+
+func (b *HexBytes) UnmarshalJSON(text []byte) error {
+	if string(text) == "null" {
+		return nil
+	}
+
+	if !isString(text) {
+		return errors.New("marshaling a JSON value that is not null or a string")
+	}
+
+	text = text[1 : len(text)-1]
+
+	if has0xPrefix(text) {
+		text = text[2:]
+	}
+
+	if len(text)%2 != 0 {
+		return errors.New("hex string had an odd length")
+	}
+
+	out := make([]byte, len(text)/2)
+
+	_, err := hex.Decode(out, text)
+	if err != nil {
+		return fmt.Errorf("error decoding hex: %w", err)
+	}
+
+	*b = out
+	return nil
+}
+
+func has0xPrefix(text []byte) bool {
+	return len(text) >= 2 && text[0] == '0' && (text[1] == 'x' || text[1] == 'X')
+}
+
+func isString(text []byte) bool {
+	return len(text) >= 2 && text[0] == '"' && text[len(text)-1] == '"'
+}
diff --git a/json/hex_test.go b/json/hex_test.go
new file mode 100644
index 0000000..8e216c2
--- /dev/null
+++ b/json/hex_test.go
@@ -0,0 +1,141 @@
+package json
+
+import (
+	"bytes"
+	"encoding/json"
+	"testing"
+)
+
+func TestMarshalJSON(t *testing.T) {
+	cases := []struct {
+		Name     string
+		Slice    HexBytes
+		Expected string
+	}{
+		{
+			Name:     "Nil slice",
+			Slice:    nil,
+			Expected: "null",
+		},
+		{
+			Name:     "Zero-length slice",
+			Slice:    []byte{},
+			Expected: `"0x"`,
+		},
+		{
+			Name:     "Typical slice",
+			Slice:    []byte{26, 43, 60},
+			Expected: `"0x1a2b3c"`,
+		},
+	}
+
+	for _, c := range cases {
+		t.Run(c.Name, func(t *testing.T) {
+			b, err := json.Marshal(HexBytes(c.Slice))
+			if err != nil {
+				t.Fatalf("unexpected error: %v", err)
+			}
+
+			actual := string(b)
+
+			if c.Expected != actual {
+				t.Errorf("expected %q, got %q", c.Expected, actual)
+			}
+		})
+	}
+}
+
+func TestUnmarshalJSONNull(t *testing.T) {
+	var b HexBytes
+	err := json.Unmarshal([]byte(`null`), &b)
+	if err != nil {
+		t.Fatalf("unexpected error; %v", err)
+	}
+
+	if b != nil {
+		t.Errorf("expected slice to be nil, but had values %s", b)
+	}
+}
+
+func TestUnmarshalJSONSuccess(t *testing.T) {
+	cases := []struct {
+		Name     string
+		JSON     string
+		Expected []byte
+	}{
+		{
+			Name:     "No characters",
+			JSON:     `""`,
+			Expected: []byte{},
+		},
+		{
+			Name:     "No hex digits",
+			JSON:     `"0x"`,
+			Expected: []byte{},
+		},
+		{
+			Name:     "Normal with 0x prefix",
+			JSON:     `"0x1a2b3c"`,
+			Expected: []byte{26, 43, 60},
+		},
+		{
+			Name:     "Normal with 0X prefix",
+			JSON:     `"0X1a2b3c"`,
+			Expected: []byte{26, 43, 60},
+		},
+		{
+			Name:     "Normal with no prefix",
+			JSON:     `"1a2b3c"`,
+			Expected: []byte{26, 43, 60},
+		},
+	}
+
+	for _, c := range cases {
+		t.Run(c.Name, func(t *testing.T) {
+			var b HexBytes
+			err := json.Unmarshal([]byte(c.JSON), &b)
+			if err != nil {
+				t.Fatalf("unexpected error: %v", err)
+			}
+
+			if !bytes.Equal(c.Expected, b) {
+				t.Errorf("expected %s, got %s", c.Expected, b)
+			}
+		})
+	}
+}
+
+func TestUnmarshalJSONFailure(t *testing.T) {
+	cases := []struct {
+		Name  string
+		JSON  string
+		Error string
+	}{
+		{
+			Name: "Odd number of hex digits",
+			JSON: `"0x1a2b3"`,
+		},
+		{
+			Name: "JSON number",
+			JSON: `45`,
+		},
+		{
+			Name: "Invalid prefix",
+			JSON: `"0Y1a2b3c"`,
+		},
+		{
+			Name: "Non-hexadecimal digits",
+			JSON: `"0x4r"`,
+		},
+	}
+
+	for _, c := range cases {
+		t.Run(c.Name, func(t *testing.T) {
+			var b HexBytes
+			err := json.Unmarshal([]byte(c.JSON), &b)
+			if err == nil {
+				t.Errorf("expected an error, but got value %s and no error", b)
+			}
+		})
+	}
+}