New bride Ronda Rousey just got a late wedding gift -- well wishes from UFC champ Amanda Nunes ... aka the chick who KO'd her into retirement.

"Congratulations to Ronda – this is a huge moment in her life," Amanda told TMZ Sports.

"I'm very happy for her ... nothing bad (to say) about Ronda Rousey."

Nunes' change of heart makes sense ... the champ's also got marriage on the mind.

But don't get it twisted -- all her attention is on throwing down with Valentina Shevchenko at UFC 215 this Saturday. 

Amanda wouldn't give us an official prediction ... but she did make 1 guarantee for the grudge match.