Robert Patrick -- Juror T-1000 ... On Gang Murder Trial

An alleged murderer is facing termination ... from an actual Terminator, because Robert Patrick -- the ole T-1000 is sitting on his jury.

The 'Terminator 2' star is serving on a gang related murder trial currently going on in Downtown Los Angeles. In the case in question ... Deon Montea Archie is accused of shooting and killing Markai Brown in 2013.

The trial's been going on for about a week now, and Patrick seems to be in good spirits. He tweeted on Wednesday ... "Starting day 7 #civicduty."