The effects of 400 Hz anodal and cathodal transcranial pulsed current stimulation of the primary motor cortex (400 Hz a-tPCS _M1, 400 Hz c-tPCS _M1) on corticospinal excitability (CSE) and corticocortical excitability (CCE) remain underexplored. This study examined the effects of 400 Hz a-tPCS _M1, 400 Hz c-tPCS _M1, and sham stimulation on CSE, CCE, and hand dexterity, providing insights for potential clinical applications in motor deficits. In this double-blinded, randomized, counterbalanced crossover trial, 26 healthy young adults completed three experimental sessions: 400 Hz a-tPCS _M1, 400 Hz c-tPCS _M1, and sham stimulation, spaced 48 hours apart. Transcranial magnetic stimulation assessed CSE and CCE, while the Purdue Pegboard Test (PPT) evaluated hand dexterity. The results showed polarity-specific effects. A single session of 400 Hz a-tPCS _M1 significantly increased CSE and improved hand dexterity, evidenced by faster PPT completion times (p < 0.05). Conversely, 400 Hz c-tPCS _M1 reduced CSE but did not influence PPT performance (p > 0.05). Sham stimulation showed no significant changes. These findings suggest that 400 Hz a-tPCS _M1 enhances motor excitability and dexterity, while 400 Hz c-tPCS _M1 selectively reduces CSE. This study lays a foundation for exploring high-frequency tPCS in clinical motor rehabilitation.