Yasiel Puig just had the best pre-game warm up of his life ... 'cause just a few hours before the Indians' game against the Red Sox -- he got sworn in as a U.S. citizen!
The MLB superstar had the ceremony Wednesday ... and it didn't take long for him to pridefully post about it on social media.
"Thank you God for this great opportunity to be an American citizen," Puig said ... adding, "ususus."
The Indians -- who are set for first pitch against Boston in Cleveland at 10 a.m. PT -- even shouted out the outfielder after the special moment, writing, "Puig our citizen"
"Congratulations to Yasiel Puig becoming a United States citizen!