Alvarado was beaten up and stopped within three one-sided rounds at the 1st Bank Centre, in Broomfield, Colorado.
From the start Rios pounded Alvarado until the ring doctor stopped the fight following the end of the third round.
Rios looked good but his opponent looked like he had nothing in the tank after the early clashes in the opening round.
"It was my best fight ever. My plan was to box from the outside and hit him with body shots.
"Our careers were on the line. I had to prove to everybody that I'm here to stay," Rios said afterwards.