The pair - who have had an on/off relationship for years - attended the Floyd Mayweather fight on Saturday night (15.09.14) and were also spotted walking hand-in-hand around the Hard Rock Hotel.
Bieber, who is a friend of the boxing champion, posted pictures of himself in the city as he prepared to attend the big fight, saying: "At the @HardRockHotelLV pool before heading over to see the Champ (sic)."
The sight of the celebrity couple together comes shortly after E! News reported that the Come And Get It hitmaker revealed to attorney Mark DiCowden that the pair had reunited.
Justin is said to have confirmed the relationship during his follow-up deposition in relation to a lawsuit filed against him by a photographer who claims he was assaulted by the singer.
The rest of the deposition was "uneventful", according to the source, who added that Justin was well-behaved throughout.
Previously, Justin shared a photograph on Instagram of Selena kissing his cheek as he hugged her, before deleting the image almost immediately.