Lady Gaga Showin Skin at Tony Bennett's Bday!

Lady Gaga looked like she was hittin' a club instead of Tony Bennett's 90th birthday tribute -- still, we're willing to bet there were no complaints.

Gaga showed up to Tony's birthday celebration Thursday night at Radio City Music Hall with a see-through black dress ... hiked right up to her crotch.

She looked awesome, but that's a dangerous outfit to wear around the elderly.