Click on photos for gallery
Zac, 26, looked dapper in a three-piece suit as he headed out for the special screening in LA. Despite arriving on his own, he happily posed with his arm around 20-year-old Halston during a group photograph.
Zac Efron and rumoured girlfriend Halston Sage at the Neighbors premiere
Halston, 20, chose a sequinned, strapless black mini dress for her moment in the spotlight, leaving her blonde hair loose around her shoulders.
The couple first met on the set of the comedy, which also stars Seth Rogen and Dave Franco.
They were first romantically linked at the start of the month, with a source telling E! News, "They were seeing each other a bit during the filming of their movie together and ever since he went to rehab they got a lot closer.
Zac Efron signs autographs for his excited fans
"Her friends love him and say he is so sweet. He's really good to her."
Halston was previously dating Glee star Chord Overstreet; the pair confirmed their split shortly after the actress was photographed at a basketball game with Zac.
With a blossoming romance and the release of his new film, things finally seem to be looking up for the actor.
Cast photo: Zac Efron and Halston Sage alongside Seth Rogen and Dave Franco
At the end of March, he was attacked in the Skid Row area of LA after he and his bodyguard became embroiled in a fight with local residents, and prior to that, Zac slipped and broke his jaw during an accident at his home. In September, it was revealed that he had completed a stint in rehab after falling in with the wrong crowd.
Neighbors — or Bad Neighbours as it will be known in the UK — follows a couple (Seth Rogen and Rose Byrne) who move into a new neighborhood with their newborn baby.
But they face a number of challenges when a partying fraternity, led by Zac's character, moves in next door.