Kai Cenat said Kevin Hart's got the height of a leprechaun ... and TMZ Sports got an immediate response from the man himself -- with Hart telling Cenat to go "f*** himself!!"

We caught up with the comedian out on the streets of NYC shortly after Cenat hopped on "First Take" with Stephen A. Smith ... an appearance in which the ESPN talent compared Kai's athletic ability to Hart's.

The streamer did not take the notion too kindly ... saying, "You just compared me to that little leprechaun? Kevin Hart? Are you serious?"

When we told the 45-year-old what was said ... he gave us a message for the internet superstar -- "Tell Cenat I said go f*** himself!"

It was all in good fun -- the two have grown close to the point where Kevin made a few appearances on the 23-year-old's live streams ... and Hart even told us, "He is dope as hell."

They first appeared together on a stream in May 2024 ... and returned in June alongside Druski to break the Twitch North American record for viewership with 712,600 concurrent watchers.

The two then appeared on Cenat's Mafiathon 2 ... a month-long stream in which Cenat broke Twitch's subscriber count.

While we're not sure when the two will appear on stream again ... fans gotta be demanding the two match up on the blacktop in a one-on-one after all this trash talk!!