In this letter, we present a new concept for normally-off AlGaN/GaN-on-Si MOS-HEMTs based on the combination of p-GaN, tri-gate and MOS structures to achieve high threshold voltage (V-TH) and low on-resistance (R-ON). The p-GaN is used to engineer the band structure and reduce the carrier density (N-s) in the tri-gate structure for a high V-TH. The gate control is mainly achieved from field-effect through the tri-gate sidewalls, and does not rely on injection of gate current. The MOS structure enables much larger gate voltages (V-G) and the effective sidewall modulation results in excellent switching performance at high switching frequencies. In addition, this concept eliminates the need for thin barriers (typical in p-GaN devices), which combined to the conduction channels formed at the tri-gate sidewalls, resulted in a smaller R-ON compared with planar p-GaN structures. The p-GaN length and tri-gate filling factor (FF) were optimized to achieve a good trade-off between high V-TH and low R-ON. The excellent channel control capability offered by the tri-gate structure led to a higher ON/OFF ratio and smaller sub-threshold slope (SS) compared to similar planar p-GaN devices. These results unveil the excellent prospects of p-GaN tri-gate MOS technology for future power electronics applications.