Serena Williams Reveals Massive Engagement Ring

Serena Williams just showed the world her new engagement ring ... and it's massive.

Williams posted the pic Thursday with new fiance and Reddit co-founder Alexis Ohanian and -- while it's not a super close-up of the rock -- you can tell she's sporting some serious bling.

The couple announced their engagement last week ... but somehow the diamond hasn't seen the light of day until now.

A Grand Slam for Ohanian.