An arrest warrant had been issued for Martin, relating to an Instagram post showing a shotgun and ammunition. He was booked into the Van Nuys Jail Tuesday afternoon, Los Angeles police said.
Martin then entered not guilty pleas in a Van Nuys courtroom to four felony counts of making criminal threats and one misdemeanor count of carrying a loaded firearm in a vehicle.
He was released on bond and is due back in court April 25 for a preliminary hearing.
Martin was charged last Tuesday. If convicted, he could be sentenced to up to six years.
Martin, who graduated from Harvard-Westlake in 2008, tagged the school, two former classmates and two of his former NFL teammates in the post.
The photo on his private but verified Instagram account had the caption "When you're a bully victim & a coward, your options are suicide or revenge".
Harvard-Westlake canceled classes last month in response to the post.
The former Miami Dolphins offensive lineman was detained by police and has since checked into a mental health facility. Police were investigating if Martin created the post or if it was the work of hackers.
Following the incident, Harvard-Westlake filed a restraining order against Martin. The order prohibits Martin from entering the campus and requires him to stay 150 yards away from school properties and facilities. He must also stay 150 yards away from Rick Commons, the president and head of the school.