Nick Young Gets Engaged on Christmas Day with Massive Rock

Nick Young saved his girlfriend's best gift for last ... asking her to marry him with a MASSIVE engagement ring!!!

The charming moment went down on Christmas morning with help from his son, Nick Jr. who filmed the whole thing. You can see Nick grab the huge ring box from a different room and walk over to his baby mama Keonna Green and said the proposal is "way overdue."

It's a pretty swaggy moment ... the entire fam -- wearing matching PJs -- saw Nick tell Keonna he wants to spend the rest of his life with her and, of course, she said yes. Nick let out a huuuuge cheer after he slipped the ring on her. He then turned to his son and said, "We did it, son!!" Nick Jr. was stoked about it too.

We're told Nick worked with Jason of Beverly Hills to come up with the perfect ring for his Keonna ... and he was really stoked with the results.

"Nick was very hands on and took his time to design the ring and plan the proposal. You can tell he really enjoyed the process and I can definitely see the connection between him and Keonna. He proposed to his soulmate," Jason Arasheben, CEO Jason of Beverly Hills tells us.

Nick and Keonna started dating in high school. They have 3 kids together ... Nick Jr., daughter Navi and daughter Nyce, who was born in July.