Abstract: A locking device for medical tube coupling includes an elongated body positionable during use adjacent an exterior surface of the medical tube coupling, a first flange carried on one end of the body, a slot formed through a thickness of the first flange in an open communication with an edge surface thereof, and a second flange carried on a longitudinally opposite end of the body and extending in a direction of the first flange. The second flange may include the same slot and/or a cavity in an inner surface thereof. The locking device may be made adjustable to lock a connection between a tube and the medical tube coupling and/or securely cap the inlet port of the medical tube coupling when tube is removed. The device may also include a third flange being disposed on the elongated body between the first and second flanges.