<p>Strong alongshore currents generate considerable instabilities due to the cross-shore velocity gradient of the mean alongshore current. However, the precise impact of these instabilities on the temporal and spatial evolution of turbulent stress and the mean longshore current in the surf zone remains unclear, and experimental validation in this regard is even more limited. This study investigates the spatiotemporal variability of turbulent stress and the mean longshore current driven by shear instabilities in the surf zone through a series of controlled laboratory experiments. The experiments were conducted in a 55 m × 34 m × 1 m deep wave basin with a plane beach slope of 1:100, subjected to monochromatic, unidirectional, and obliquely incident waves with a large incident angle of 30°. Nine wave conditions were tested, covering a range of wave heights from 2.4 cm to 5.0 cm and periods from 1.0 s to 2.0 s, with wave steepness ranging from 0.004 8 to 0.028 8. The results indicate that shear instabilities significantly influence the very low frequency (VLF) turbulent stress and the cross-shore velocity profiles of alongshore currents in both spatial and temporal domains. The maximum VLF turbulent stress values were observed near the breaker line, highlighting this region as a hotspot for significant mixing. The spatiotemporal variations of turbulent stress and mean alongshore currents were found to be the primary drivers for eddy generation and development. The study provides the experimental validation of the synchronization between shear instability induced turbulent stress variations and eddy formation in the surf zone. The maximum alongshore current velocity increased over time, with significant adjustments observed in the rear shear region of the velocity profile.</p>