A model rocket tube coupler is a small section of body tube material fits snugly with a body tube. A tube coupler, as name implies, is typically used to join two sections of body tube together to form a longer section of body tube.

The added advantage of the tube coupler is that is provides additional rigidity to the body tube. For example, a 30" BT-50 body tube would not be sturdy enough for flight, but two 15" pieces joined by a BT-50 tube coupler would be. Sometimes a tube couple is used simply to reinforce a body tube, or in the repair of a torn or bent body tube.