The legendary skier just got engaged to her hockey superstar boyfriend, P.K. Subban, last week but she kept the ring on the down low ... until Monday night.

As you can see, the stone itself is BIGGER THAN HER KNUCKLE ... and while the price of the ring is still unknown, safe to say it ain't cheap! Good thing Subban is in the middle of an 8-year, $72 million contract!

Subban previously told Vogue he picked the emerald because green is Lindsey's favorite color -- and it also happens to be his birthstone ... so it all worked out.

Plus, it matches an emerald green necklace he gave her for Christmas.

As for the wedding, Vonn has said the two are in no hurry ... this could be one of those super long engagement things, but both seem to be okay with that.

In the meantime, enjoy that rock!!!