The tube plug comprises a substantially cylindrical sheath closed at one end and having a relatively smooth outer surface and a central cavity. An expandable shell is received within the central cavity of the sheath. An expander member is adapted to expand the shell from within by being axially translated within the shell, the expander member also being adapted to grip the inner surface of the shell to retain the expander member in the expanded position. The outside surface of the shell is preferably provided with a plurality of lands. A second embodiment of the tube plug comprises a sleeve, a closed shell member adapted to be inserted within the sleeve, and an expander member contained within the shell. Upon axial translation of the expander member within the shell, the shell is expanded into contact with the sleeve and the sleeve is expanded into contact with the tube, thereby sealing the tube.