People magazine says the 53-year-old Louganis will marry paralegal Johnny Chaillot.
The four-time gold medalist is the only man to win consecutive Olympic titles in springboard and platform diving, in 1984 at Los Angeles and 1988 at Seoul.
After his diving career ended, Louganis revealed he was gay in 1994 and announced he was HIV-positive a year later.
Louganis is helping Olympic hopefuls as an athlete mentor for USA Diving. He's also been featured as a coach on ABC's reality diving competition "Splash."