diff --git a/backend/src/domain/relay/types/relaystate.rs b/backend/src/domain/relay/types/relaystate.rs
index e651d27..f77859e 100644
--- a/backend/src/domain/relay/types/relaystate.rs
+++ b/backend/src/domain/relay/types/relaystate.rs
@@ -1,11 +1,13 @@
 /// State of a relay (on or off).
 ///
 /// Represents the binary state of a relay channel in the Modbus controller.
-#[derive(Debug, Clone, Copy, PartialEq, Eq)]
+#[derive(Debug, Clone, Copy, PartialEq, Eq, serde::Serialize, serde::Deserialize)]
 pub enum RelayState {
     /// Relay is energized (circuit closed).
+    #[serde(rename = "on")]
     On,
     /// Relay is de-energized (circuit open).
+    #[serde(rename = "off")]
     Off,
 }
 
diff --git a/specs/001-modbus-relay-control/tasks.md b/specs/001-modbus-relay-control/tasks.md
index ae5f6aa..eda7f91 100644
--- a/specs/001-modbus-relay-control/tasks.md
+++ b/specs/001-modbus-relay-control/tasks.md
@@ -225,7 +225,7 @@
   - **File**: src/domain/relay.rs
   - **Complexity**: Low | **Uncertainty**: Low
 
-- [ ] **T020** [P] [US1] [TDD] Implement RelayState enum
+- [x] **T020** [P] [US1] [TDD] Implement RelayState enum
   - Enum: On, Off
   - Implement Display, Debug, Clone, Copy, PartialEq, Eq, serde::Serialize/Deserialize
   - **File**: src/domain/relay.rs