Kid Cudi is testifying about his infamous car fire-bombing … and he says he knows Diddy had something to do with it, even though Diddy denied it to his face during a meeting where Diddy came off like a Marvel supervillain.

The rapper testified Thursday in Diddy’s criminal … telling the jury his Porsche went up in flames outside his house in January 2012, shortly after he broke things off with Cassie.

Kid Cudi says he wasn’t at home when he got a call from his dog’s babysitter, informing him his car was on fire. He says he rushed home and found his car damaged.

Kid Cudi told jurors he reached out to Diddy a few days after the incident, “because I knew he had something to do with it.” Cudi said he told Diddy they needed to meet up and talk. He said Diddy had wanted to talk to him and things were getting out of hand, so he decided to meet.

Cudi says he went to see Diddy at SoHo House in West Hollywood … with Diddy’s security guard D-Roc escorting him into a room where Diddy was standing there like a “Marvel supervillain.”

Kid Cudi says they chopped it up about Cassie, and Diddy told him he thought they were homies and that Cassie was his girl. Cudi says he told Diddy he thought they had broken up.

Cudi told the jury Diddy was so calm during the meeting it was “weird” … and that Cassie came in the room too and told him she and Cudi had fallen in love.

Kid Cudi testified he confronted Diddy about the car bombing, asking him “What about my car?” He said Diddy responded, “I don’t know what you’re talking about.”