D4vd‘s friend Neo Langston finally went in front of a grand jury after being arrested in Montana for failing to appear as a witness.

Neo showed up Wednesday to a Los Angeles courthouse where a grand jury has been convened since November to gather information on the death of Celeste Rivas … whose dismembered corpse was found last September inside D4vd’s Tesla in Hollywood.

We got Neo on his way out of the courthouse … and he was mum as our photog peppered him with questions.

We caught up with Beth Silverman, the deputy D.A. presenting the case in Los Angeles, on her way into the courthouse … and she was all business.

Neo was in front of the grand jury for about 30 to 40 minutes … and he has since left the courtroom.

Given Neo’s friendship with D4vd, who is a suspect in Celeste’s murder, it’s surprising Silverman spent only 40 minutes with him … the prosecutor has been aggressive with questioning witnesses.

TMZ broke the story … Neo, a popular streamer, was cuffed by a small army of cops in Helena, MT, after an L.A. judge signed off on an arrest warrant.

Neo, who had been squirreled away at his mom’s house, was then transported to L.A. and booked into LAPD’s Metropolitan Detention Center on $60,000 bail before bonding out.

Now, it appears Neo was asked questions in the Rivas case. Multiple witnesses have already testified.