Jameis Winston is adding yet another sport to his training arsenal ... this time it's boxing -- and the NFL QB is doing it with a personal punching lesson from Keith Thurman!!!
The new Saints quarterback hit up the legendary St. Pete Boxing Club in St. Petersburg, Fla. on Thursday to get in a boxing sesh with One Time -- and the vid of the training is pretty intense.
Thurman showed Jameis how to throw punches with knockout power ... and by the end of it -- the former No. 1 overall pick looked all right hitting the heavy bag!