The marriage licence was issued 10 days ago for Manson and Afton Elaine Burton, the Associated Press reports.
Ms Burton moved to Corcoran, California, nine years ago in order to be nearer Manson's prison, it adds.
Manson is serving a life sentence for the murders of seven people and one unborn child in Los Angeles in 1969.
Their victims included pregnant actress Sharon Tate, wife of film director Roman Polanski.
Ms Burton, who calls herself Star, told AP that she and Manson would marry next month. The licence is reportedly valid for 90 days.
"Y'all can know that it's true... It's going to happen," she told the agency.
"I love him," she added.
The cult leader and his followers, known as the Manson Family, stabbed and shot seven people in Los Angeles over two nights in August 1969 in an attempt to start a race war.
Manson and three women accomplices were sentenced to death for the killings, but that was commuted in 1972 when California temporarily outlawed the death penalty.
In 2012, Manson was refused parole by a Californian prison panel - it was the 12th time he had made a bid for freedom.
He is not eligible to apply for parole again until 2027.