He really is Groot.
Vin Diesel -- the voice of the resurrected plant hero -- really got into the shrubby spirit Monday at the "Avengers: Infinity War" premiere in Hollywood ... wearing part of Groot on his suit!
The premiere was insane -- and even had a special cameo from Stan Lee ... who's been battling health and legal issues. He got a hero's welcome.
Other stars include ... Chadwick Boseman, Scarlett Johansson, Robert Downey Jr., Chris Hemsworth, Gwyneth Paltrow and Star-Lord himself, Chris Pratt.
The one star who didn't make the premiere -- Chris Evans. Guessing it has to do with the fact he's starring in a play on Broadway at the moment.
As for the reviews ... so far, pretty solid. Kevin Smith LOVED it.
"I am overwhelmed. The movie is epic beyond compare."
You excited yet?!