Gomez and Bieber's backstage kiss
The couple have been dating on-and-off for two years, but in a radio interview earlier this month Selena, 20, said she was single and available "in every way".
The pair did not sit together at the Billboard awards - where Justin, 19 was booed on stage accepting the award for Social Artist - but a picture shows Selena backstage with her arm around Justin kissing him tenderly on the cheek.
Justin - who has come under attack in the media of late for a string of controversial events on his world tour - looked visibly upset on stage when he was booed and defending himself as a serious artist, insisting "I am not a gimmick".
Selena gave a sizzling performance of her new single Come And Get It in a skimpy gold dress.
The couple split for the latest time at the beginning of the year, but sparked rumours of a reconciliation on several occasion with reports Selena had met up with Justin while he was on tour in Europe.
Last month Justin posted a picture of Selena with her arms around him with the caption: "You've been makin' music for too long babe come cuddle."