In the field of sensorless back electromotive force (EMF) estimation for Permanent Magnet Synchronous Motors (PMSMs), this paper introduces a novel super-twisting sliding mode observer with adaptive gain features. This observer is designed to address the common challenges of chattering, inadequate adaptability, and complex tuning that often affect traditional sliding mode observers. Initially, a time-varying gain function was developed based on the dynamic characteristics of the sliding surface. This function was subsequently optimized using a finite-time stability mechanism, which simplifies the system design to a single tunable parameter that automatically adjusts all gains to accommodate varying operational speed conditions. Furthermore, to address the issues of high susceptibility to disturbances, limited robustness, and slow dynamic responses associated with the speed outer-loop PI controllers in Field-Oriented Control (FOC) systems, this adaptive observer was integrated into an anti-disturbance speed control strategy. This strategy significantly enhances the motor's resilience against parameter perturbations and sudden changes in load torque. The effectiveness of the proposed approach in delivering accurate rotor position estimations and improved disturbance resistance in sensorless control scenarios was validated through comprehensive simulation results.