A once-promising baseball player used a bat engraved with his name to kill his father, uncle and a bystander at the family’s Southern California home in 2015, a jury found on Thursday.

Brandon Martin — a 27-year-old former first-round pick of the Tampa Bay Rays — was found guilty of first-degree murder in the brutal killings.

Prosecutors said the ex-minor leaguer used a wooden bat to beat his disabled dad, Michael Martin, 64; his uncle, Ricky Anderson, 58; and an alarm system installer, Barry Swanson, 62, who had been in the home at the time.

Martin apparently began unraveling after he was picked by the Rays in 2011 in the first round of the MLB draft — and once punched his father in the face and put his mother in a headlock, the Los Angeles Times reported.

He played three seasons with the Rays’ farm team before being released on March 26, 2015.

Shortly before the murders, Martin had confessed to police that he choked his mother and threatened her with scissors, the Times reported.

He was held for two days at a mental health facility — and then immediately went to his family home when he was released on Sept. 17, 2015.

There, he attacked his dad, uncle and Swanson, who was at the home to install an alarm system “because the family feared Brandon Martin,” according to the Riverside County district attorney’s office.

Martin’s cousin discovered the bloodbath and called 911. His dad and Swanson were pronounced dead at the scene and his uncle died two days later.

Corona Police spotted Martin in Swanson’s stolen pickup truck about a day after the killings — leading to a chase that ended with Martin leaving the truck and running.

At one point in the chase, the ballplayer broke into a home and jumped from a second-story window, then fought with a police dog before being cuffed, the DA’s office said.

Jurors will now decide whether to recommend life in prison or the death penalty.