In this paper, a speed-adaptive reduced-order observer for sensorless vector control of doubly fed induction generators (DFIGs) is proposed. The observer is a simulation of the rotor current dynamic model with feedback of the estimation error and a speed-adaptation loop. Feedback and adaptation gains are designed based on the closed-loop observer model. A parameter sensitivity analysis reveals that this observer is robust against machine parameter variations in the normal operating regions. Simulation results demonstrate desired steady-state and dynamic performance of this sensorless control approach for DFIG-based variable-speed wind turbines.