The 26-year-old singer, who famously became a figure of hate during her stint on the 2010 series, announced her plans to marry Brad Alphonso on Twitter yesterday (July 8).
"Just woken up with a huge smile on my face," she wrote. "Can't believe that we are engaged!! @Brad_Alphonso."
Her fiancé, who recently appeared half-naked in Madonna's 'Girl Gone Wild' video, replied: "@katiewaissel24 Holy s**t!!!!! It's for real !!! Love it!!!!"
Waissel later tweeted photos of herself and Alphonso getting matching heart tattoos on their ring fingers, as well as several more shots of them together.
The reality TV star has previously dated the likes of Capital FM DJ Greg Burns and Michael Sophocles of The Apprentice.
Speaking last year, Waissel admitted that she could not understand why she received so much "hate" on The X Factor.
"I know you can't please everyone, but what went on, with proper hatred and death threats, that was weird," she said.
"It was scary. But I just couldn't understand why. Usually to get that reaction someone needs to commit a really awful crime or say something totally outrageous."