LSU's Leonard Fournette in Street Fight vs. Teammate ... Or Horseplay??

LSU running back Leonard Fournette -- a Heisman Trophy favorite -- was involved in a shocking street fight with one of his teammates ... but LSU says it was just innocent horseplay.

It all went down Tuesday outside Tiger Stadium in Baton Rouge -- when Fournette squared up against offensive lineman Garrett Brumfield.

FYI, Fournette is 6'1", 235 lbs ... Brumfield is 6'3", 305 lbs.

In the video, obtained by TMZ Sports, the two men throw some heavy shots -- and it appears they were trying to hurt each other.

But LSU Associate Athletic Director of Communications Michael Bonnette tells TMZ Sports the team has spoken to both players and have determined it was all an intense game of slap boxing.

"A game where they basically have to touch the other person twice."

The clip seems to tell a different story -- especially with the power behind one of the overhand rights Fournette tries to land.

But Bonnette insists there is no real animosity between the two players.

"What you don't see is when the video cuts off, they hug it out. They're not mad at each other. No one was injured."