Frey, 31, is marrying Robert Hernandez, a neighbor who works in law enforcement, said her lawyer, Gloria Allred, confirming a report on People magazine's Web site.
Hernandez, who was not involved in the Peterson case, proposed after taking Frey and her two young children to Disneyland. The couple met in October 2005.
The newlyweds will continue living in the Fresno area, Allred said.
Frey, a massage therapist who dated Peterson in 2002, was the prosecution's star witness against Peterson during his trial on charges of killing his pregnant wife, Laci, and their unborn son on Christmas Eve that year.
Prosecutors used taped conversations between Frey and Peterson to show how Peterson lied in the weeks after his wife vanished and continued to romance Frey.
Peterson was convicted and sentenced to death in 2004. He remains on death row at San Quentin State Prison.