The former couple - who split up in February 2009 after he assaulted her in a car - hugged when he won the Best Male Video prize at the event at the Staples Centre, and she appeared to peck him on the cheek to congratulate him.
Rihanna herself picked up the biggest award of the night, Video of the Year, for `We Found Love`.
She recently confessed to still having feelings for Chris and said she found it hard to act normally around him whenever they saw each other.
The 24-year-old pop star said: "We went to a mutual friend`s party on a yacht.
"It`s awkward because I still love him. My stomach drops and I have to maintain this poker face and not let it get to the outer part of me.
"I have to maintain that and suppress it and interpret that and understand that is not going to go away.
"That is peaceful. When you don`t understand those feelings you can make a lot of mistakes."
One Direction were the big winners at the VMAs picking up three prizes at the ceremony - Most Share-Worthy Video, Best Pop Video and Best New Artist.
Other artists to take home gongs included Demi Lovato, who won Best Video With A Message for `Skyscraper`, and Nicki Minaj, who received the Best Female Video accolade for `Starships`.