diff --git a/esphome/components/json/json_util.cpp b/esphome/components/json/json_util.cpp
index 7f0fb4442a..79375b861b 100644
--- a/esphome/components/json/json_util.cpp
+++ b/esphome/components/json/json_util.cpp
@@ -15,7 +15,7 @@ static const char *const TAG = "json";
 static SpiRamAllocator global_json_allocator;
 #endif
 
-std::string build_json(const json_build_t &f) {
+SerializationBuffer<> build_json(const json_build_t &f) {
   // NOLINTBEGIN(clang-analyzer-cplusplus.NewDeleteLeaks) false positive with ArduinoJson
   JsonBuilder builder;
   JsonObject root = builder.root();
diff --git a/esphome/components/json/json_util.h b/esphome/components/json/json_util.h
index 5dbeed8b76..73847c2bb8 100644
--- a/esphome/components/json/json_util.h
+++ b/esphome/components/json/json_util.h
@@ -131,7 +131,8 @@ using json_parse_t = std::function<bool(JsonObject)>;
 using json_build_t = std::function<void(JsonObject)>;
 
 /// Build a JSON string with the provided json build function.
-std::string build_json(const json_build_t &f);
+/// Returns SerializationBuffer for stack-first allocation; implicitly converts to std::string.
+SerializationBuffer<> build_json(const json_build_t &f);
 
 /// Parse a JSON string and run the provided json parse function if it's valid.
 bool parse_json(const std::string &data, const json_parse_t &f);
diff --git a/esphome/components/mqtt/mqtt_client.cpp b/esphome/components/mqtt/mqtt_client.cpp
index e7364f3406..acef275d59 100644
--- a/esphome/components/mqtt/mqtt_client.cpp
+++ b/esphome/components/mqtt/mqtt_client.cpp
@@ -564,8 +564,8 @@ bool MQTTClientComponent::publish(const char *topic, const char *payload, size_t
 }
 
 bool MQTTClientComponent::publish_json(const char *topic, const json::json_build_t &f, uint8_t qos, bool retain) {
-  std::string message = json::build_json(f);
-  return this->publish(topic, message.c_str(), message.length(), qos, retain);
+  auto message = json::build_json(f);
+  return this->publish(topic, message.c_str(), message.size(), qos, retain);
 }
 
 void MQTTClientComponent::enable() {