The couple has been dating on and off since 2007 and had remained friends throughout the years, according to People Magazine.
On Saturday, Phelps posted a picture to Instagram with a caption that read "She said yes."
His fiancee shared the happy news on her account as well.
The couple had their photos taken by Phelps' training partner and fellow Olympian Allison Schmitt, who posted her own photo of the "soon to be Mr. & Mrs. Phelps."