diff --git a/src/json-pack/json/JsonDecoder.ts b/src/json-pack/json/JsonDecoder.ts
index 7522fe960c..79d0a8d7f1 100644
--- a/src/json-pack/json/JsonDecoder.ts
+++ b/src/json-pack/json/JsonDecoder.ts
@@ -26,7 +26,7 @@ const escapedCharReplacer = (char: string) => {
   return char;
 };
 
-// Starts with "data:application/octet-stream;base64," - 64 61 74 61 3a 61 70 70 6c 69 63 61 74 69 6f 6e 2f 6f 63 74 65 74 2d 73 74 72 65 61 6d 3b 62 61 73 65 36 34 2c
+// Starts with data:application/octet-stream;base64, - 64 61 74 61 3a 61 70 70 6c 69 63 61 74 69 6f 6e 2f 6f 63 74 65 74 2d 73 74 72 65 61 6d 3b 62 61 73 65 36 34 2c
 const hasBinaryPrefix = (u8: Uint8Array, x: number) =>
   u8[x] === 0x64 &&
   u8[x + 1] === 0x61 &&
@@ -66,6 +66,44 @@ const hasBinaryPrefix = (u8: Uint8Array, x: number) =>
   u8[x + 35] === 0x34 &&
   u8[x + 36] === 0x2c;
 
+// Matches "data:application/cbor,base64;9w=="
+const isUndefined = (u8: Uint8Array, x: number) =>
+  // u8[x++] === 0x22 &&  // "
+  // u8[x++] === 0x64 &&  // d
+  u8[x++] === 0x61 && // a
+  u8[x++] === 0x74 && // t
+  u8[x++] === 0x61 && // a
+  u8[x++] === 0x3a && // :
+  u8[x++] === 0x61 && // a
+  u8[x++] === 0x70 && // p
+  u8[x++] === 0x70 && // p
+  u8[x++] === 0x6c && // l
+  u8[x++] === 0x69 && // i
+  u8[x++] === 0x63 && // c
+  u8[x++] === 0x61 && // a
+  u8[x++] === 0x74 && // t
+  u8[x++] === 0x69 && // i
+  u8[x++] === 0x6f && // o
+  u8[x++] === 0x6e && // n
+  u8[x++] === 0x2f && // /
+  u8[x++] === 0x63 && // c
+  u8[x++] === 0x62 && // b
+  u8[x++] === 0x6f && // o
+  u8[x++] === 0x72 && // r
+  u8[x++] === 0x2c && // ,
+  u8[x++] === 0x62 && // b
+  u8[x++] === 0x61 && // a
+  u8[x++] === 0x73 && // s
+  u8[x++] === 0x65 && // e
+  u8[x++] === 0x36 && // 6
+  u8[x++] === 0x34 && // 4
+  u8[x++] === 0x3b && // ;
+  u8[x++] === 0x39 && // 9
+  u8[x++] === 0x77 && // w
+  u8[x++] === 0x3d && // =
+  u8[x++] === 0x3d && // =
+  u8[x++] === 0x22; // "
+
 const findEndingQuote = (uint8: Uint8Array, x: number): number => {
   const len = uint8.length;
   let char = uint8[x];
@@ -172,10 +210,19 @@ export class JsonDecoder implements BinaryJsonDecoder {
     const uint8 = reader.uint8;
     const char = uint8[x];
     switch (char) {
-      case 34: // "
-        return uint8[x + 1] === 0x64 // d
-          ? this.tryReadBin() || this.readStr()
-          : this.readStr();
+      case 34: {
+        // "
+        if (uint8[x + 1] === 0x64) {
+          // d
+          const bin = this.tryReadBin();
+          if (bin) return bin;
+          if (isUndefined(uint8, x + 2)) {
+            reader.x = x + 35;
+            return undefined;
+          }
+        }
+        return this.readStr();
+      }
       case 91: // [
         return this.readArr();
       case 102: // f
diff --git a/src/json-pack/json/JsonEncoder.ts b/src/json-pack/json/JsonEncoder.ts
index b7f8cf35b8..1506d46b56 100644
--- a/src/json-pack/json/JsonEncoder.ts
+++ b/src/json-pack/json/JsonEncoder.ts
@@ -32,6 +32,9 @@ export class JsonEncoder implements BinaryJsonEncoder, StreamingBinaryJsonEncode
             return this.writeObj(value as Record<string, unknown>);
         }
       }
+      case 'undefined': {
+        return this.writeUndef();
+      }
       default:
         return this.writeNull();
     }
@@ -41,6 +44,35 @@ export class JsonEncoder implements BinaryJsonEncoder, StreamingBinaryJsonEncode
     this.writer.u32(0x6e756c6c); // null
   }
 
+  public writeUndef(): void {
+    const writer = this.writer;
+    const length = 35;
+    writer.ensureCapacity(length);
+    // Write: "data:application/cbor,base64;9w=="
+    const view = writer.view;
+    let x = writer.x;
+    view.setUint32(x, 0x22_64_61_74); // "dat
+    x += 4;
+    view.setUint32(x, 0x61_3a_61_70); // a:ap
+    x += 4;
+    view.setUint32(x, 0x70_6c_69_63); // plic
+    x += 4;
+    view.setUint32(x, 0x61_74_69_6f); // atio
+    x += 4;
+    view.setUint32(x, 0x6e_2f_63_62); // n/cb
+    x += 4;
+    view.setUint32(x, 0x6f_72_2c_62); // or,b
+    x += 4;
+    view.setUint32(x, 0x61_73_65_36); // ase6
+    x += 4;
+    view.setUint32(x, 0x34_3b_39_77); // 4;9w
+    x += 4;
+    view.setUint16(x, 0x3d_3d); // ==
+    x += 2;
+    writer.uint8[x++] = 0x22; // "
+    writer.x = x;
+  }
+
   public writeBoolean(bool: boolean): void {
     if (bool) this.writer.u32(0x74727565); // true
     else this.writer.u8u32(0x66, 0x616c7365); // false
diff --git a/src/json-pack/json/__tests__/JsonDecoder.spec.ts b/src/json-pack/json/__tests__/JsonDecoder.spec.ts
index b8b9d4e049..d8946a6a42 100644
--- a/src/json-pack/json/__tests__/JsonDecoder.spec.ts
+++ b/src/json-pack/json/__tests__/JsonDecoder.spec.ts
@@ -27,6 +27,22 @@ describe('null', () => {
   });
 });
 
+describe('undefined', () => {
+  test('undefined', () => {
+    const encoder = new JsonEncoder(new Writer());
+    const encoded = encoder.encode(undefined);
+    const decoded = decoder.read(encoded);
+    expect(decoded).toBe(undefined);
+  });
+
+  test('undefined in array', () => {
+    const encoder = new JsonEncoder(new Writer());
+    const encoded = encoder.encode({foo: [1, undefined, -1]});
+    const decoded = decoder.read(encoded);
+    expect(decoded).toEqual({foo: [1, undefined, -1]});
+  });
+});
+
 describe('boolean', () => {
   test('true', () => {
     const data = Buffer.from('true', 'utf-8');
diff --git a/src/json-pack/json/__tests__/JsonEncoder.spec.ts b/src/json-pack/json/__tests__/JsonEncoder.spec.ts
index ff7ff4034c..3c5d031b83 100644
--- a/src/json-pack/json/__tests__/JsonEncoder.spec.ts
+++ b/src/json-pack/json/__tests__/JsonEncoder.spec.ts
@@ -19,6 +19,20 @@ describe('null', () => {
   });
 });
 
+describe('undefined', () => {
+  test('undefined', () => {
+    const encoded = encoder.encode(undefined);
+    const txt = Buffer.from(encoded).toString('utf-8');
+    expect(txt).toBe('"data:application/cbor,base64;9w=="');
+  });
+
+  test('undefined in object', () => {
+    const encoded = encoder.encode({foo: undefined});
+    const txt = Buffer.from(encoded).toString('utf-8');
+    expect(txt).toBe('{"foo":"data:application/cbor,base64;9w=="}');
+  });
+});
+
 describe('boolean', () => {
   test('true', () => {
     assertEncoder(true);