"It is with much sadness that we report the passing of Scott McKenzie," Gary and Raylene Hartman wrote on the site.
McKenzie died in Los Angeles Saturday.
"Scott had been very ill recently and passed away in his home after two weeks in hospital," they said.
McKenzie had suffered since 2010 from Guillain-Barre syndrome, a disease that affects the nervous system.
"It has been our pleasure to maintain this Web site over the past 15 years and this is the hardest update of them all," the Hartmans said. "Farewell our much loved and wonderful friend."
They said McKenzie may have suffered a heart attack early this month. "Staff did not want him to leave the hospital, but he wanted to be at home," the Web site said.
McKenzie was born Philip Wallach Blondheim Jan. 10, 1939, in Jacksonville, Fla., and grew up in Montreat, N.C.
After his father died, his mother moved to Alexandria, Va., where he started singing in local clubs. He also became friends with John Phillips, and they performed as The Journeymen.
Phillips later became "Papa John" with the 1960s vocal group the Mamas & the Papas.
McKenzie told The Washington Post in a 1977 interview he didn't want to join the Mamas & the Papas because "I was trying to see if I could do something by myself. And I didn't think I could take that much pressure."
Phillips wrote and co-produced "San Francisco" for McKenzie and played guitar on the recording, released May 13, 1967.
The song became an instant hit, reaching No. 4 on the Billboard Hot 100 and drawing as many as 100,000 young people to San Francisco's Haight-Ashbury neighborhood that summer.
McKenzie's song was also a top hit in Britain and other countries, selling more than 7 million copies globally, "The Book of Golden Discs" said.
McKenzie followed "San Francisco" with "Like an Old Time Movie," also written and produced by Phillips.
His debut album, "The Voice of Scott McKenzie," in 1967 was followed by "Stained Glass Morning" in 1970, his final album.
In 1986, McKenzie started singing with a new version of the Mamas & the Papas. He co-wrote the 1988 Beach Boys No. 1 single "Kokomo" with Terry Melcher, Mike Love and John Phillips.