Two sources tell E! News the duo are "back on" and are enjoying their "casual" relationship.
The on-again couple have been hanging out daily since their sighting last week at Katy's Memorial Day party.
"They are just hanging out," one of the sources explained. "They are not putting a label on it."
While the two have apparently been spending nights together, they haven't made a public appearance since their split in March.
Mayer opened to Ellen DeGeneres just weeks later saying, "Coupling is a tricky thing."
The two began seeing one another in August of 2012 and were first spotted on a romantic date night at the Chateau Marmont.
Meanwhile, Mayer's little black book includes Jennifer Aniston, Jessica Simpson and Minka Kelly.
Katy's rep could not be reached for comment.
Are you happy to see these two back together? Weigh in below.