diff --git a/backend/src/domain/relay/entity.rs b/backend/src/domain/relay/entity.rs
new file mode 100644
index 0000000..2307e96
--- /dev/null
+++ b/backend/src/domain/relay/entity.rs
@@ -0,0 +1,63 @@
+//! Relay entity representing a relay aggregate in the domain model.
+
+use super::types::{RelayId, RelayState};
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use crate::domain::relay::controler::ControllerError;
+
+    #[test]
+    fn test_relay_new_creates_relay() {
+        // Test: Relay::new(RelayId(1), RelayState::Off, None) creates relay
+        let relay_id = RelayId::new(1).unwrap();
+        let relay = Relay::new(relay_id, RelayState::Off, None);
+
+        assert_eq!(relay.id(), relay_id);
+        assert_eq!(relay.state(), RelayState::Off);
+        assert_eq!(relay.label(), None);
+    }
+
+    #[test]
+    fn test_relay_toggle_flips_state() {
+        // Test: relay.toggle() flips state
+        let relay_id = RelayId::new(1).unwrap();
+        let mut relay = Relay::new(relay_id, RelayState::Off, None);
+
+        // Toggle from Off to On
+        relay.toggle();
+        assert_eq!(relay.state(), RelayState::On);
+
+        // Toggle from On to Off
+        relay.toggle();
+        assert_eq!(relay.state(), RelayState::Off);
+    }
+
+    #[test]
+    fn test_relay_turn_on_sets_state_to_on() {
+        // Test: relay.turn_on() sets state to On
+        let relay_id = RelayId::new(1).unwrap();
+        let mut relay = Relay::new(relay_id, RelayState::Off, None);
+
+        relay.turn_on();
+        assert_eq!(relay.state(), RelayState::On);
+
+        // Calling turn_on when already on should keep it on
+        relay.turn_on();
+        assert_eq!(relay.state(), RelayState::On);
+    }
+
+    #[test]
+    fn test_relay_turn_off_sets_state_to_off() {
+        // Test: relay.turn_off() sets state to Off
+        let relay_id = RelayId::new(1).unwrap();
+        let mut relay = Relay::new(relay_id, RelayState::On, None);
+
+        relay.turn_off();
+        assert_eq!(relay.state(), RelayState::Off);
+
+        // Calling turn_off when already off should keep it off
+        relay.turn_off();
+        assert_eq!(relay.state(), RelayState::Off);
+    }
+}
diff --git a/backend/src/domain/relay/mod.rs b/backend/src/domain/relay/mod.rs
index 4e63f39..26748e6 100644
--- a/backend/src/domain/relay/mod.rs
+++ b/backend/src/domain/relay/mod.rs
@@ -9,3 +9,5 @@ pub mod repository;
 pub mod types;
 /// Controller error types for relay operations.
 pub mod controler;
+/// Relay entity representing the relay aggregate.
+pub mod entity;
diff --git a/specs/001-modbus-relay-control/tasks.md b/specs/001-modbus-relay-control/tasks.md
index eda7f91..4d81a07 100644
--- a/specs/001-modbus-relay-control/tasks.md
+++ b/specs/001-modbus-relay-control/tasks.md
@@ -231,7 +231,7 @@
   - **File**: src/domain/relay.rs
   - **Complexity**: Low | **Uncertainty**: Low
 
-- [ ] **T021** [US1] [TDD] Write tests for Relay aggregate
+- [x] **T021** [US1] [TDD] Write tests for Relay aggregate
   - Test: Relay::new(RelayId(1), RelayState::Off, None) creates relay
   - Test: relay.toggle() flips state
   - Test: relay.turn_on() sets state to On