Their friend Amber Rose was quick to congratulate the pair, writing on Instagram: ''Congrats to my girl @lira_galore and @richforever. Love is a beautiful thing.''
Mercer - who is believed to be 21 and uses the name Lira Galore on social media - was previously linked to Drake, Justin Bieber and Diddy. Three years ago she tweeted: ''Ima Marry Rick Ross. My fat boy lol (sic).''
However, Ross - whose real name is William Leonard Roberts II - and Lira only started dating this year with the couple first linked in March after he split from Ming Lee.
Meanwhile, earlier this year, Ross' bitter rival 50 Cent was ordered to pay $5 million in damages to Rick's ex Lastonia Leviston after he posted a sex tape of her and a former boyfriend online in order to humiliate the rapper. Ross has a daughter with Lastonia.