People magazine says the 53-year-old Louganis will marry paralegal Johnny Chaillot.
The four-time gold medallist 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."