He was sentenced to death in 1969 along with four of his "Manson Family" disciples for having led the killing of seven people
The 82-year-old was moved from Corcoran State Prison in Kings County, California, to a hospital 60 miles (100 kilometers) to the south in Bakersfield, according to celebrity news portal TMZ.
A state corrections official refused to comment on the reports and would confirm only that Manson had not died, but the Los Angeles Times cited an unnamed source who said Manson was "seriously ill," without elaborating.
Manson headed an apocalyptic cult that committed murders in upscale, mostly white neighborhoods of Los Angeles in order to blame the crimes on African Americans, in the hope of sparking what he termed a "Helter Skelter" race war.
He has been in prison for more than four decades after the 1969 killings, which included the brutal murder of director Roman Polanski's actress wife, Sharon Tate, who was heavily pregnant.
He was sentenced to death in 1969 along with four of his "Manson Family" disciples for having led the killing of seven people, but their sentences were later commuted to life in prison.
Manson, who was not actually present but ordered the killings, applied for parole in 2012 but was denied release and is not eligible to apply again until 2027.
He was granted permission in 2014 to wed then 26-year-old Afton Elaine Burton, described in US media as a slender brunette, who had been visiting him in prison.
But the 90-day license expired without the ceremony going ahead.
He was married twice before he was jailed, first to Rosalie Jean Willis from 1955 to 1958, and later to Candy Stevens between 1959 and 1963.