Machine Gun Kelly made an unexpected appearance at Lil Durk's federal trial ... showing up at the courthouse to listen in on the proceedings.

Sources with direct knowledge tell TMZ ... MGK showed up to a downtown Los Angeles court Monday and sat in an overflow room, where he listened to the trial as it unfolded.

We're told MGK's appearance was unexpected, though he and Durk have some history together ... MGK joined Durk onstage during his 2023 Amazon Music Live performance in Los Angeles, playing guitar as Durk performed "The Voice."

Durk has been behind bars since his October 2024 arrest in connection with an alleged murder-for-hire plot.

Federal prosecutors allege Durk sought revenge after his friend and fellow rapper King Von was fatally shot in 2020 outside an Atlanta hookah lounge.

Prosecutors claim members of Durk's crew later targeted rapper Quando Rondo in California in 2022 ... but Quando survived the shooting while his cousin, Lul Pab, was killed.

Durk has denied wrongdoing and remains in federal custody.