The boxer was not a happy man when he found out about the unfollow.
Floyd Mayweather has 16.4 million followers on Instagram.
Justin Bieber is not included in that group.
The Biebs used to be one of his followers and part of his "Money Team," walking into the ring with him at a fight in 2012.
TMZ reports that Bieber unfollowed Mayweather because he wants to "reset boundaries" as he gets involved in his new church.
Justin unfollowed Floyd on Instagram as part of the resetting of boundaries, and our sources say Floyd went "insane, nuclear." He lashed out at Justin, called him a "traitor" because Floyd had stuck with Justin during his meltdown when everyone was attacking Bieber.
Sources connected with Floyd say the boxer continues to be incensed at Justin ... screaming incensed.
Mayweather is set to meet Connor McGregor in boxing's most-hyped fight of the decade on Saturday night.