An anonymous source told entertainment news site TMZ that Manson was rushed to Bakersfield hospital in central California three days ago, escorted by five policeman. "It's not going to get any better for him," the source said.
Manson was hospitalized in January for severe intestinal bleeding and needed surgery, but doctors deemed him too weak and sent him back to prison.
The 83-year-old has been behind bars for more than four decades.
In 1971, he and four followers were sentenced to death for the August 1969 killing spree that left seven dead - including Tate, wife of filmmaker Roman Polanski.
Their sentences were later commuted to life imprisonment.