TMZ Sports has learned ... the New York Giants legend rolled dice at the Seminole Hard Rock Hotel & Casino in Hollywood, FL for hours on Thursday night -- and he appeared to win BIG.

One witness tells us ... Manning showed up before 9:30 PM and stayed past 2:30 AM -- albeit with a short break for food at some point -- seemingly raking in cash throughout the evening.

In fact, in video we obtained, you can see that after one run in particular ... he celebrated like a madman, high-fiving his tablemates before sharing a huge embrace with one of his pals.

We're told Eli seemed to have around $40,000 in chips in front of him ... while some of the guys he was playing with had over $100K.

Of course, it ain't like Eli exactly needs the dough ... after all, the former signal-caller made over $250 MILLION in his NFL career alone.