This paper considers nonlinear free vibration of a spinning tether. The tether is a part of a tethered satellite system spinning at a constant rate about its nominal longitudinal axis. Using Hamilton’s principle, the governing equations of motion are derived for the station-keeping phase retaining the nonlinear terms up to the third order. These equations are discretized by the assumed-modes method and then analyzed by applying the method of averaging. When the tether has moderately high nominal tension, averaging with two time variables leads to a closed-form solution, which describes the dependence of the frequency content on the initial amplitude parameters. In the case of very low nominal tension, averaging with only one of the two time variables is permissible to obtain accurate solutions. One can then obtain the steady state and the limit steady state solutions, both of which represent circular whirling motion like a skip-rope. Analytical expressions governing the amplitudes of the whirling motions are presented in the paper. In the absence of damping, the general transverse motion is quasi-periodic, but it can be periodic if the initial conditions are chosen correctly. Numerical investigations reveal that the material damping associated with longitudinal motion drives the steady state to the limit steady state.