Mike Piazza -- Ultra Rare $25k Watch ... Hall of Fame Election Gift

Mike Piazza's timepiece game just got stronger ... courtesy of a SWEET GIFT worth $25k from the CEO of a luxury watchmaker ... TMZ Sports has learned.

The gift giving went down in Miami at a party thrown by Swiss watchmaker Ulysse Nardin and luxury lifestyle magazine Haute Living ... and the soon-to-be Hall of Famer was in attendance.

Piazza -- who's a friend of U.N.'s CEO -- was given a Ulysse Nardin Midnight Express Special Edition watch -- 1 of only 100 in existence -- to celebrate his election into Cooperstown.

It's good to be one of the greatest baseball players of all time.