An estimator of the wind speed of a wind turbine coupled to a generator is proposed in this paper. Wind speed enters into the generator dynamics through a highly nonlinear function; hence, we are confronted with a difficult problem of estimation of a nonlinearly parameterized system. To solve this problem, we use the technique of immersion and invariance, recently introduced in the literature. It is assumed that the rotor speed and electrical torque of the generator are measured, which is the case for the machines typically used in this application. The result is of interest for the design of controllers of maximum power extraction, where the knowledge of the wind speed is necessary to express the control objective as a speed tracking problem. Detailed computer simulations are presented to assess the performance of the proposed estimator.