IBF welterweight champ Errol Spence Jr. is coming after Floyd Mayweather's title as most iced-out fighter alive ... 'cause the boxing star just copped an incredible "Man Down" chain for $400k!
TMZ Sports has learned ... the undefeated stud wanted to immortalize his signature slogan in diamonds and spared no expenses to get the job done.
Spence hit up Iceman Nick to make the custom piece ... and the specs are incredible:
The pendant (worth around $150k on its own) is made with 55 carats of VS1 diamonds on white and rose gold while the infinity link Cuban chain (valued around $250k) has 60 carats of VS1 diamonds on 1000 grams of rose gold.