UFC legend Cain Velasquez spent Monday night in jail ... TMZ Sports has confirmed the 39-year-old has been arrested and booked on an attempted murder charge.

Details surrounding the specifics of his arrest are unclear -- though law enforcement sources tell us the former fighter was involved in a shooting in San Jose at around 3 PM near a local high school.

In the alleged incident, one person was shot while another was arrested. Nothing further surrounding the circumstances of the situation has been released.

Jail records show Velasquez is still in custody as of Tuesday morning ... and has a court date set for Wednesday at noon.

Velasquez -- who was born and raised in Northern California -- is one of the best heavyweight MMA fighters in the history of the sport. During his reign, Cain beat everyone from Brock Lesnar to Junior dos Santos.

After parting ways following his last loss to Francis Ngannou, Cain spent several months with WWE (he's a huge wrestling fan).

Cain trains at the famed American Kickboxing Academy (AKA) gym with Daniel Cormier and Khabib Nurmagomedov, among other MMA stars.

Before getting into mixed martial arts, Velasquez was a star collegiate wrestler at Arizona State University. He was a 2x All-American.