Posting a picture of him and Katy looking loved up on his Instagram account on Friday, the One Direction star wrote, "She said yes! It's happening!"
Although his fans' hearts must be breaking, the photo has already racked up over 434,000 likes.
Niall first jokingly popped the question to his Teenage Dream crush on 21 October, after Katy performed her new single Roar on the X Factor.
Tweeting at the time, the One Direction star confessed, "I love you @katyperry! Thank you!" before asking her, "Ps. Will you marry me?"
The pair have been friends ever since Katy appeared as a guest judge on the X Factor in 2010, and was so impressed by Niall's solo audition that she put him through to the boot camp stage of the competition, where One Direction were formed as a group.
The two popstars then showed off their close bond at the 2012 VMAs, where the songstress planted a kiss on Niall's lips on stage for all to see.
After the show Niall confessed, "Katy's super-hot," adding, "And she's really cool as well."
Now it seems Niall wants to take their relationship to another level, even appointing his groomsmen for the big day, and fans won't be surprised to find out who he has picked.
Sharing another photo on Instagram, Niall and Katy this time posed with the rest of One Direction and captioned the snap, "Me and my groomsmen and my lovely wife."