The pair was spotted sitting courtside at a Los Angeles Lakers basketball game earlier this month (Apr14) and now editors at Eonline.com report the couple was there on a date.
A source says, "They are definitely an item. They were seeing each other a bit during filming of their movie together and ever since he went to rehab (in 2013), they got a lot closer.
"Her friends love him and say he is so sweet. He's really good to her."
Efron and Sage are set to spend a lot more time together as they promote Neighbors - the new comedy is due for release next month (May14).
The relationship news emerges days after it was revealed that Sage had recently ended her short-lived romance with Glee star Chord Overstreet, who she was first spotted with in January (14).
Efron's exes include his High School Musical co-star Vanessa Hudgens, who he dated from 2005 to 2010.