The 22-year-old joined United in 2010 in a deal worth £7.4m but has failed to hold down a regular place at Old Trafford and was shipped out to Besiktas after just one year at the club.
He spent most of his spell at the Turkish team on the sidelines after tearing his anterior cruciate ligament while on international duty with Portugal's under-21 side, and is yet to make an appearance for United this season.
However, he will now return to his homeland in a bid to get games under his belt and prove his worth to Red Devils boss Sir Alex Ferguson.
Bebe told the Rio Ave club website www.rioave-fc.pt: "I'm ready to play and pleased to wear the Rio Ave shirt.
"I'll give it everything while I'm here and try to help the team to continue occupying the places that they have been in the table."
He added: "Obviously something went wrong in the last phase of my career, I had a serious injury which thankfully I have fully recovered well from.
"I'm fine physically, I was always training and playing. It's good to come
here and I want to show what I can do."
Rio Ave are fifth in the Portuguese top flight on 18 points, 14 behind leaders Benfica.