diff --git a/include/xgboost/json.h b/include/xgboost/json.h
index a5872ec3a..77ca6a510 100644
--- a/include/xgboost/json.h
+++ b/include/xgboost/json.h
@@ -1,5 +1,5 @@
 /**
- * Copyright 2019-2023 by XGBoost Contributors
+ * Copyright 2019-2024, XGBoost Contributors
  */
 #ifndef XGBOOST_JSON_H_
 #define XGBOOST_JSON_H_
@@ -42,7 +42,8 @@ class Value {
     kBoolean,
     kNull,
     // typed array for ubjson
-    kNumberArray,
+    kF32Array,
+    kF64Array,
     kU8Array,
     kI32Array,
     kI64Array
@@ -173,7 +174,11 @@ class JsonTypedArray : public Value {
 /**
  * @brief Typed UBJSON array for 32-bit floating point.
  */
-using F32Array = JsonTypedArray<float, Value::ValueKind::kNumberArray>;
+using F32Array = JsonTypedArray<float, Value::ValueKind::kF32Array>;
+/**
+ * @brief Typed UBJSON array for 64-bit floating point.
+ */
+using F64Array = JsonTypedArray<double, Value::ValueKind::kF64Array>;
 /**
  * @brief Typed UBJSON array for uint8_t.
  */
@@ -457,9 +462,9 @@ class Json {
   Json& operator[](int ind)                 const { return (*ptr_)[ind]; }
 
   /*! \brief Return the reference to stored Json value. */
-  Value const& GetValue() const & { return *ptr_; }
-  Value const& GetValue() &&      { return *ptr_; }
-  Value&       GetValue() &       { return *ptr_; }
+  [[nodiscard]] Value const& GetValue() const& { return *ptr_; }
+  Value const& GetValue() && { return *ptr_; }
+  Value& GetValue() & { return *ptr_; }
 
   bool operator==(Json const& rhs) const {
     return *ptr_ == *(rhs.ptr_);
@@ -472,7 +477,7 @@ class Json {
     return os;
   }
 
-  IntrusivePtr<Value> const& Ptr() const { return ptr_; }
+  [[nodiscard]] IntrusivePtr<Value> const& Ptr() const { return ptr_; }
 
  private:
   IntrusivePtr<Value> ptr_{new JsonNull};
diff --git a/include/xgboost/json_io.h b/include/xgboost/json_io.h
index 3a73d170a..ce3d25c37 100644
--- a/include/xgboost/json_io.h
+++ b/include/xgboost/json_io.h
@@ -142,6 +142,7 @@ class JsonWriter {
 
   virtual void Visit(JsonArray  const* arr);
   virtual void Visit(F32Array  const* arr);
+  virtual void Visit(F64Array const*) { LOG(FATAL) << "Only UBJSON format can handle f64 array."; }
   virtual void Visit(U8Array  const* arr);
   virtual void Visit(I32Array  const* arr);
   virtual void Visit(I64Array  const* arr);
@@ -244,7 +245,8 @@ class UBJReader : public JsonReader {
  */
 class UBJWriter : public JsonWriter {
   void Visit(JsonArray const* arr) override;
-  void Visit(F32Array  const* arr) override;
+  void Visit(F32Array const* arr) override;
+  void Visit(F64Array const* arr) override;
   void Visit(U8Array  const* arr) override;
   void Visit(I32Array  const* arr) override;
   void Visit(I64Array  const* arr) override;
diff --git a/src/common/json.cc b/src/common/json.cc
index 21be2a5bc..2887eeccf 100644
--- a/src/common/json.cc
+++ b/src/common/json.cc
@@ -1,11 +1,12 @@
 /**
- * Copyright 2019-2023, XGBoost Contributors
+ * Copyright 2019-2024, XGBoost Contributors
  */
 #include "xgboost/json.h"
 
 #include <array>             // for array
 #include <cctype>            // for isdigit
 #include <cmath>             // for isinf, isnan
+#include <cstdint>           // for uint8_t, uint16_t, uint32_t
 #include <cstdio>            // for EOF
 #include <cstdlib>           // for size_t, strtof
 #include <cstring>           // for memcpy
@@ -72,15 +73,16 @@ void JsonWriter::Visit(JsonNumber const* num) {
 }
 
 void JsonWriter::Visit(JsonInteger const* num) {
-  char i2s_buffer_[NumericLimits<int64_t>::kToCharsSize];
+  std::array<char, NumericLimits<int64_t>::kToCharsSize> i2s_buffer_;
   auto i = num->GetInteger();
-  auto ret = to_chars(i2s_buffer_, i2s_buffer_ + NumericLimits<int64_t>::kToCharsSize, i);
+  auto ret =
+      to_chars(i2s_buffer_.data(), i2s_buffer_.data() + NumericLimits<int64_t>::kToCharsSize, i);
   auto end = ret.ptr;
   CHECK(ret.ec == std::errc());
-  auto digits = std::distance(i2s_buffer_, end);
+  auto digits = std::distance(i2s_buffer_.data(), end);
   auto ori_size = stream_->size();
   stream_->resize(ori_size + digits);
-  std::memcpy(stream_->data() + ori_size, i2s_buffer_, digits);
+  std::memcpy(stream_->data() + ori_size, i2s_buffer_.data(), digits);
 }
 
 void JsonWriter::Visit(JsonNull const* ) {
@@ -143,8 +145,10 @@ std::string Value::TypeStr() const {
       return "Null";
     case ValueKind::kInteger:
       return "Integer";
-    case ValueKind::kNumberArray:
+    case ValueKind::kF32Array:
       return "F32Array";
+    case ValueKind::kF64Array:
+      return "F64Array";
     case ValueKind::kU8Array:
       return "U8Array";
     case ValueKind::kI32Array:
@@ -262,10 +266,11 @@ bool JsonTypedArray<T, kind>::operator==(Value const& rhs) const {
   return std::equal(arr.cbegin(), arr.cend(), vec_.cbegin());
 }
 
-template class JsonTypedArray<float, Value::ValueKind::kNumberArray>;
-template class JsonTypedArray<uint8_t, Value::ValueKind::kU8Array>;
-template class JsonTypedArray<int32_t, Value::ValueKind::kI32Array>;
-template class JsonTypedArray<int64_t, Value::ValueKind::kI64Array>;
+template class JsonTypedArray<float, Value::ValueKind::kF32Array>;
+template class JsonTypedArray<double, Value::ValueKind::kF64Array>;
+template class JsonTypedArray<std::uint8_t, Value::ValueKind::kU8Array>;
+template class JsonTypedArray<std::int32_t, Value::ValueKind::kI32Array>;
+template class JsonTypedArray<std::int64_t, Value::ValueKind::kI64Array>;
 
 // Json Number
 bool JsonNumber::operator==(Value const& rhs) const {
@@ -708,6 +713,8 @@ Json UBJReader::ParseArray() {
     switch (type) {
       case 'd':
         return ParseTypedArray<F32Array>(n);
+      case 'D':
+        return ParseTypedArray<F64Array>(n);
       case 'U':
         return ParseTypedArray<U8Array>(n);
       case 'l':
@@ -797,6 +804,10 @@ Json UBJReader::Parse() {
         auto v = this->ReadPrimitive<float>();
         return Json{v};
       }
+      case 'D': {
+        auto v = this->ReadPrimitive<double>();
+        return Json{v};
+      }
       case 'S': {
         auto str = this->DecodeStr();
         return Json{str};
@@ -825,10 +836,6 @@ Json UBJReader::Parse() {
         Integer::Int i = this->ReadPrimitive<char>();
         return Json{i};
       }
-      case 'D': {
-        LOG(FATAL) << "f64 is not supported.";
-        break;
-      }
       case 'H': {
         LOG(FATAL) << "High precision number is not supported.";
         break;
@@ -882,6 +889,8 @@ void WriteTypedArray(JsonTypedArray<T, kind> const* arr, std::vector<char>* stre
   stream->push_back('$');
   if (std::is_same<T, float>::value) {
     stream->push_back('d');
+  } else if (std::is_same_v<T, double>) {
+    stream->push_back('D');
   } else if (std::is_same<T, int8_t>::value) {
     stream->push_back('i');
   } else if (std::is_same<T, uint8_t>::value) {
@@ -910,6 +919,7 @@ void WriteTypedArray(JsonTypedArray<T, kind> const* arr, std::vector<char>* stre
 }
 
 void UBJWriter::Visit(F32Array const* arr) { WriteTypedArray(arr, stream_); }
+void UBJWriter::Visit(F64Array const* arr) { WriteTypedArray(arr, stream_); }
 void UBJWriter::Visit(U8Array const* arr) { WriteTypedArray(arr, stream_); }
 void UBJWriter::Visit(I32Array const* arr) { WriteTypedArray(arr, stream_); }
 void UBJWriter::Visit(I64Array const* arr) { WriteTypedArray(arr, stream_); }
diff --git a/tests/cpp/common/test_json.cc b/tests/cpp/common/test_json.cc
index 155cf04ad..72163efd7 100644
--- a/tests/cpp/common/test_json.cc
+++ b/tests/cpp/common/test_json.cc
@@ -639,6 +639,40 @@ TEST(Json, TypedArray) {
       ASSERT_EQ(arr[i + 8], i);
     }
   }
+
+  {
+    Json f64{Object{}};
+    auto array = F64Array();
+    auto& vec = array.GetArray();
+    // Construct test data
+    vec.resize(18);
+    std::iota(vec.begin(), vec.end(), 0.0);
+    // special values
+    vec.push_back(std::numeric_limits<double>::epsilon());
+    vec.push_back(std::numeric_limits<double>::max());
+    vec.push_back(std::numeric_limits<double>::min());
+    vec.push_back(std::numeric_limits<double>::denorm_min());
+    vec.push_back(std::numeric_limits<double>::quiet_NaN());
+
+    static_assert(
+        std::is_same_v<double, typename std::remove_reference_t<decltype(vec)>::value_type>);
+
+    f64["f64"] = std::move(array);
+    ASSERT_TRUE(IsA<F64Array>(f64["f64"]));
+    std::vector<char> out;
+    Json::Dump(f64, &out, std::ios::binary);
+
+    auto loaded = Json::Load(StringView{out.data(), out.size()}, std::ios::binary);
+    ASSERT_TRUE(IsA<F64Array>(loaded["f64"]));
+    auto const& result = get<F64Array const>(loaded["f64"]);
+
+    auto& vec1 = get<F64Array const>(f64["f64"]);
+    ASSERT_EQ(result.size(), vec1.size());
+    for (std::size_t i = 0; i < vec1.size() - 1; ++i) {
+      ASSERT_EQ(result[i], vec1[i]);
+    }
+    ASSERT_TRUE(std::isnan(result.back()));
+  }
 }
 
 TEST(UBJson, Basic) {
@@ -694,6 +728,7 @@ TEST(UBJson, Basic) {
   }
 }
 
+
 TEST(Json, TypeCheck) {
   Json config{Object{}};
   config["foo"] = String{"bar"};
diff --git a/tests/cpp/test_serialization.cc b/tests/cpp/test_serialization.cc
index 0b65220ab..283a56fc5 100644
--- a/tests/cpp/test_serialization.cc
+++ b/tests/cpp/test_serialization.cc
@@ -60,7 +60,7 @@ void CompareJSON(Json l, Json r) {
     }
     break;
   }
-  case Value::ValueKind::kNumberArray: {
+  case Value::ValueKind::kF32Array: {
     auto const& l_arr = get<F32Array const>(l);
     auto const& r_arr = get<F32Array const>(r);
     ASSERT_EQ(l_arr.size(), r_arr.size());
@@ -69,6 +69,15 @@ void CompareJSON(Json l, Json r) {
     }
     break;
   }
+  case Value::ValueKind::kF64Array: {
+    auto const& l_arr = get<F64Array const>(l);
+    auto const& r_arr = get<F64Array const>(r);
+    ASSERT_EQ(l_arr.size(), r_arr.size());
+    for (size_t i = 0; i < l_arr.size(); ++i) {
+      ASSERT_NEAR(l_arr[i], r_arr[i], kRtEps);
+    }
+    break;
+  }
   case Value::ValueKind::kU8Array: {
     CompareIntArray<U8Array>(l, r);
     break;