diff --git a/esphome/components/dsmr/dsmr.h b/esphome/components/dsmr/dsmr.h
index fafcf62b87..dc81ba9b2a 100644
--- a/esphome/components/dsmr/dsmr.h
+++ b/esphome/components/dsmr/dsmr.h
@@ -64,6 +64,9 @@ class Dsmr : public Component, public uart::UARTDevice {
   void dump_config() override;
 
   void set_decryption_key(const char *decryption_key);
+  // Remove before 2026.8.0
+  ESPDEPRECATED("Pass .c_str() - e.g. set_decryption_key(key.c_str()). Removed in 2026.8.0", "2026.2.0")
+  void set_decryption_key(const std::string &decryption_key) { this->set_decryption_key(decryption_key.c_str()); }
   void set_max_telegram_length(size_t length) { this->max_telegram_len_ = length; }
   void set_request_pin(GPIOPin *request_pin) { this->request_pin_ = request_pin; }
   void set_request_interval(uint32_t interval) { this->request_interval_ = interval; }
diff --git a/tests/components/dsmr/common.yaml b/tests/components/dsmr/common.yaml
index 038bf2806b..d11ce37d59 100644
--- a/tests/components/dsmr/common.yaml
+++ b/tests/components/dsmr/common.yaml
@@ -1,4 +1,13 @@
+esphome:
+  on_boot:
+    then:
+      - lambda: |-
+          // Test deprecated std::string overload still compiles
+          std::string key = "00112233445566778899aabbccddeeff";
+          id(dsmr_instance).set_decryption_key(key);
+
 dsmr:
+  id: dsmr_instance
   decryption_key: 00112233445566778899aabbccddeeff
   max_telegram_length: 1000
   request_pin: ${request_pin}