diff --git a/esphome/components/json/json_util.cpp b/esphome/components/json/json_util.cpp
index 9ce346aa8a..80d48431cb 100644
--- a/esphome/components/json/json_util.cpp
+++ b/esphome/components/json/json_util.cpp
@@ -114,7 +114,7 @@ SerializationBuffer<> JsonBuilder::serialize() {
   }
 
   // Needs heap allocation - reallocate and serialize again with exact size
-  result.reallocate_heap(size);
+  result.reallocate_heap_(size);
   serializeJson(doc_, result.data_writable(), size + 1);
   return result;
 }
diff --git a/esphome/components/json/json_util.h b/esphome/components/json/json_util.h
index 4c51a61ddf..73beb05c92 100644
--- a/esphome/components/json/json_util.h
+++ b/esphome/components/json/json_util.h
@@ -89,9 +89,15 @@ template<size_t STACK_SIZE = 768> class SerializationBuffer {
   /// Set actual size after serialization (must not exceed allocated size)
   void set_size(size_t size) { size_ = size; }
 
+  /// Implicit conversion to std::string for backward compatibility
+  operator std::string() const { return std::string(buffer_, size_); }  // NOLINT(google-explicit-constructor)
+
+ private:
+  friend class JsonBuilder;  ///< Allows JsonBuilder::serialize() to call reallocate_heap_()
+
   /// Reallocate to heap buffer with new size (for when stack buffer is too small)
-  /// This invalidates any previous buffer content
-  void reallocate_heap(size_t size) {
+  /// This invalidates any previous buffer content. Used by JsonBuilder::serialize().
+  void reallocate_heap_(size_t size) {
     delete[] heap_buffer_;
     heap_buffer_ = new char[size + 1];
     buffer_ = heap_buffer_;
@@ -99,10 +105,6 @@ template<size_t STACK_SIZE = 768> class SerializationBuffer {
     buffer_[0] = '\0';
   }
 
-  /// Implicit conversion to std::string for backward compatibility
-  operator std::string() const { return std::string(buffer_, size_); }  // NOLINT(google-explicit-constructor)
-
- private:
   char stack_buffer_[STACK_SIZE];
   char *heap_buffer_{nullptr};
   char *buffer_;