Katie Holmes: Yes, That's a Ring ... on THAT Hand!

Katie Holmes casually posed for pics in NYC Thursday night as if no one would notice the gold ring on her ring finger ... on her left hand.

Katie was hanging with Maggie Gyllenhaal and Camila Alves -- and while there's no huge diamond set in it, the ring is getting a ton of attention ... possibly (okay, definitely) because of her longtime rumored boyfriend Jamie Foxx.

Jamie was also spotted wearing a gold band on his left hand the night before ... also in NYC.

But Foxx's rep tells TMZ they're not engagement or wedding rings. We're told Katie's ring isn't even a promise ring ... not from Jamie, anyway.

Still, things that make you go hmmm ...