In this paper, a hybrid sliding-mode observer is first proposed to identify a nonmatching time-varying parameter that affects the speed sensorless series dc motor through an unknown-state dependent vector. Based on the measurable current and input voltage, the unknown speed and nonmatching parameter can be accurately and robustly estimated without filtering effect in a well-controlled condition. The identified parameter is then used to enhance the speed estimation performance in the presence of external disturbance. The stability analyses for the proposed observers are given, and the results are experimentally tested on a series dc motor.