Johnson (17-2-1) became the first UFC flyweight champion when he defeated Joseph Benavidez by decision at UFC 152. Johnson last fought on Jan. 26, defeating John Dodson by unanimous decision.
Moraga (13-1) is 2-0 inside the UFC Octagon with a knockout over Ulysses Gomez and a submission victory over Chris Cariaso.
The bout was first reported by The Las Vegas Review Journal on Monday, and a later confirmed by UFC officials.
The flyweight title bout will headline the card expected to take place at The Joint at the Hard Rock Hotel and Casino. The main card will air live on FX, while preliminary bouts will be showcased on Facebook and FUEL TV.
For more fight announcements, cancellations and rumors, check out MMAWeekly.com's UFC and MMA rumors section.