Kid Cudi Arrives at Diddy Trial in NYC While Smoking a Cigarette

Kid Cudi will testify today at Diddy's federal sex trafficking trial ... and, it looks like he's taking a few nicotine hits to steady himself before he does.

We recorded the rapper getting out of his car and walking into the Manhattan courthouse Thursday ... a serious expression on his face. He didn't say anything to the gathered crowd -- using his mouth to puff on a cigarette instead.

Cudi quickly turns the corner out of view in the clip -- before resurfacing inside the courthouse, where he can be seen through a window ... taking off his jacket to go through a security checkpoint, speaking to a guard.

Judge Arun Subramanian ruled earlier in the trial that Cudi can tell the jury about an incident of arson after Diddy allegedly threatened him over his romance with Cassie.

Cassie previously testified Diddy went through her phone and found emails between her and Cudi ... becoming so angry he lunged at her with a corkscrew.

In a lawsuit, Cassie claims Diddy told her he was going to blow up Cudi's car ... and, "around that time, Kid Cudi's car exploded in his driveway" the lawsuit stated.