Caitlyn Jenner in Peek-a-Boob Dress

Caitlyn Jenner put the Victoria's Secret models on notice in NYC -- she's down to flash just as much side boob as they do.

Perhaps inspired by Kendall Jenner's VS Fashion Show debut, Caitlyn hit the after-party at TAO and let ... some of it hang out.

She's 66, but those ta-tas are damn near brand new.