The 32-year-old NBAer revealed he popped the question to Blair Bashen on Monday ... getting on one knee at Surfer's Point Beach in Ventura.

FYI -- DG and Blair have been dating for 5 years ... and Green finally decided to make things official on their anniversary.

So ... what took so long?? Get this ...

"Sorry it took so long babe! just wanted to be able to get you the ring you deserved and couldn’t afford it until now 😂 "

Worth noting ... Green has reportedly made more than $67 MILLION during his career so far -- and is currently in the middle of a 2-year $30 mil contract with the Lakers.

So, either he's just teasing, or Blair just got one MASSIVE rock!!!

Green continued ... "love you with all my heart and Im so lucky to have had you by my side these past 5 years ... many more years to come! Here’s to forever!"