Kate Upton's Ass Out In NYC ... See-Through Dress with MLB Fiance

Kate Upton was upstaged by her own ass at her 24th bday party in NYC Wednesday night -- hitting up the bash with her MLB star fiance Justin Verlander in a see-through outfit.

The Detroit Tigers pitcher was all smiles as he walked Upton into the bash at The Blond lounge -- and it's obvious why.

Huge stars like Bella Hadid and Nick Jonas also hit up the party ... wearing WAY more clothes.