The 33-year-old pop star first announced the news in a video posted by the Billboard Music Awards on Instagram.
Earlier this month, the singers were seen donning 80s-inspired outfits while filming the "Pretty Girls" music video.
Azalea, 24, told MTV about her relationship with Spears.
"She's super nice. She came to my house and had lunch and I didn't know what to expect, and we actually really got along," Iggy said. "I've been texting her, annoying her with all my ideas for the video and she's been a really good sport about it, so I'm really excited"
The highly anticipated track will appear on Britney's ninth studio album and be released May 5, giving fans enough time to familiarize themselves with the lyrics just before seeing it live on the award show May 17 at 8 p.m. on ABC.