WWE star Rusev might be a tough guy in the ring but he's a softie for his real-life girlfriend Lana ... because he asked her to marry him. 

Sources close to the couple tell TMZ Rusev proposed to his WWE manager gf last month at their Nashville home. We're told Rusev proposed in the couple's swimming pool and filmed the whole thing.

Judging by the size of the rock ... we're guessing it was pretty easy for Lana to say yes. 

It's interesting, Rusev recently got engaged to WWE Diva Summer Rae on SmackDown but we're told it was just for the show ... Lana takes the title as his real life fiancé.