Bird, 24, appeared in Brighton Municipal Court in Massachusetts to answer charges of witness intimidation and threatening to commit a crime. Court documents show that he told his girlfriend "tell anyone about this, I don't care where you are I'll find you and kill you."
Bird had previously pleaded not guilty to charges of assault and battery on a household member, assault and battery with a dangerous weapon, kidnapping, and strangulation or suffocation.
According to prosecutors, the woman had told police that Bird strangled her and threw her against a wall after they got into an argument at Bird's residence. She said Bird would strangle her until she went "limp," allow her to catch her breath and begin choking her again.
She also said that Bird kicked her multiple times in the stomach and dragged her away from the door when she tried to leave, before shutting her into a bathroom.
The woman told police she was able to leave after Bird passed out with "seizure-like" symptoms.
The Celtics had said in a statement in September that the matter will be handled by the NBA rather than the team.
"Most importantly, our thoughts are with the victim of this incident. The Celtics organization deplores domestic violence of any kind, and we are deeply disturbed by the allegations against Jabari Bird," the team said at the time. "Pursuant to Domestic Violence Policy in the NBA's labor agreement, matters of this kind are handled by the League Office, not the team, and so the Celtics will be working with both the league and local authorities to assist in their ongoing investigations. The team will have no further comment at this time."
The Celtics drafted Bird with the 56th pick in the 2017 draft. The shooting guard spent his rookie season on a two-way contract before being signed to the active roster last summer.
If the Celtics were to waive Bird, they would have to pay his guaranteed $1.35 million salary for this season. By allowing the league to handle the matter, his contract could potentially be voided, depending on how the legal proceedings play out. For a Celtics team near the luxury-tax line, those savings could be substantial, particularly if they were to sign a player to replace Bird on the roster.
Bird's next scheduled court date is Feb. 13.
Information from The Associated Press was used in this report.