diff --git a/esphome/components/dallas/dallas_component.cpp b/esphome/components/dallas/dallas_component.cpp
index 3610e79447..17412b08c9 100644
--- a/esphome/components/dallas/dallas_component.cpp
+++ b/esphome/components/dallas/dallas_component.cpp
@@ -235,7 +235,7 @@ float DallasTemperatureSensor::get_temp_c() {
 
   return temp / 128.0f;
 }
-std::string DallasTemperatureSensor::unique_id() { return "dallas-" + str_upper_case(format_hex(this->address_)); }
+std::string DallasTemperatureSensor::unique_id() { return "dallas-" + str_lower_case(format_hex(this->address_)); }
 
 }  // namespace dallas
 }  // namespace esphome
diff --git a/esphome/core/helpers.cpp b/esphome/core/helpers.cpp
index 5f29abe579..11a2b1acb0 100644
--- a/esphome/core/helpers.cpp
+++ b/esphome/core/helpers.cpp
@@ -316,8 +316,8 @@ template<int (*fn)(int)> std::string str_ctype_transform(const std::string &str)
   std::transform(str.begin(), str.end(), result.begin(), [](unsigned char ch) { return fn(ch); });
   return result;
 }
-std::string str_lower_case(const std::string &str) { return str_ctype_transform<std::toupper>(str); }
-std::string str_upper_case(const std::string &str) { return str_ctype_transform<std::tolower>(str); }
+std::string str_lower_case(const std::string &str) { return str_ctype_transform<std::tolower>(str); }
+std::string str_upper_case(const std::string &str) { return str_ctype_transform<std::toupper>(str); }
 std::string str_snake_case(const std::string &str) {
   std::string result;
   result.resize(str.length());