Randy Couture -- For My Son's Birthday We'll Be In The Octagon Together

MMA legend, Randy Couture, is a master of the octagon ... so most people wouldn't want to be in the ring with him ... unless it was covered in frosting like the one he gave his son for his birthday.

Randy's son, Ryan Couture, celebrated his his 33rd birthday at Andiamo Italian Steakhouse inside the D Casino in Las Vegas on Thursday night.

Pops gave Ryan -- who is also an MMA fighter -- a cake resembling the ring both are known for.

We're told things got a little messy when Randy smeared cake across his face ... but it was all in good fun.