Ex-NFL Player Jonathan Martin Surrenders to Police

Former NFL lineman Jonathan Martin turned himself into police on Tuesday ... TMZ has learned.

Martin has been charged with 4 felonies -- 4 counts of making criminal threats. He's also been charged with 1 misdemeanor for carrying a loaded firearm in a vehicle.

Martin has pled not guilty.

The surrender follows a bizarre incident on February 23, where Martin posted a photo of a shotgun and ammo with the message, "When you're a bully victim & a coward, your options are suicide, or revenge."

According to the docs, the criminal threats were made against the San Diego Chargers' Mike Pouncey, and Martin's former Dolphins teammate Richie Incognito ... as well as 2 of his old high school teammates at Harvard Westlake. The school was put on lockdown the day Martin posted the shotgun pic.

Prosecutors say Martin faces up to 6 years in prison if convicted.

Martin spent less than 2 hours in jail before being booked and released.