More trouble for former UFC star BJ Penn ... the fighting legend is reportedly under police investigation for DUI after crashing his truck in Hawaii last week.
Police in Hilo tell Big Island Now, Penn was driving a black Toyota Tacoma pickup at around 7 p.m. on Feb. 7 when it crashed along a main highway.
Cops say a 911 caller indicated 41-year-old Penn had been speeding ... and say the crash was gnarly, with the truck losing control and flipping into an area near a mall.