The 38-year-old singer has bid 415,000 pounds for the 1.3-acre White Rock Island, a 22-mile boat ride from the Los Angeles shoreline, according to friends, reported Daily Star.
The outcrop, a favourite dive site for tourists, is owned by Hollywood director Michael Caffrey, who has only used it for film, TV and commercial shoots so far.
Home to a variety of exotic birds, White Rock also boasts regular crystal clear night skies so Williams, who is famous for his sky-gazing activities, can indulge his passion.
It already has planning permission so the former Take That heartthrob can build a luxury retreat on it.
Williams fell in love with White Rock while UFO hunting more than a year ago with his wife Ayda Field, 33, who is expecting their first child.
"It's a dream location for him and means he, Ayda and the baby won't have to check into any local hotels where he's always recognised," said a friend.
Williams, who attended a UFO convention in Nevada, claims to have seen three flying saucers.
"He's in negotiations and he's thrilled at the prospect of buying White Rock.
Ayda's hooked on his UFO kick," said a source close to the singer.