John Cena busted out a ring IN THE RING ... popping the question to fellow WWE superstar Nikki Bella -- and like her brother-in-law, Daniel Bryan, she said, "YES! YES! YES!"

Cena whipped out the massive Tiffany's diamond after he and Nikki beat up The Miz and Maryse in a co-ed tag team match at WrestleMania 33.

Here's the thing ... Cena didn't use her "ring name" -- he proposed with her real name, "Stephanie Nicole Garcia-Colace" -- leading most people to think the proposal was real and not a scripted bit. 

John and Nikki have been dating in real life since 2012.

After the event, Cena tweeted about the proposal.