Ronda Rousey has a brand new tag team partner -- a 6-year-old girl -- and together they get some adorably violent revenge on a wrestling event security guard!!!  

Here's the deal ... Rousey was in the crowd at the WWE SmackDown Live in Vegas on Tuesday when she was spotted by a 6-year-old fan who wanted a photo. 

An arena security guard stopped the girl from getting to Ronda (basically, he did his job) -- but the little girl was devastated and began to bawl her eyes out. 

Ronda watched the situation unfold and beelined it to little Hailey Danielson -- where the UFC legend immediately put the guard in a full nelson and let the kid exact sweet fist revenge!!! 

They laughed. They hugged. They took pics. Ronda even invited the kid to sit next to her in the front row. 

And now the security guard will forever know ... NO ONE MESSES WITH HAILEY!!!!