Theodore Ralph Fernandes, 51, of Fontana took a plea deal at West Valley Superior Courthouse and accepted responsibility for two counts of sexual penetration by a foreign object against the victim's will and one count of lewd act on a child age 14, according to court records.
As part of the agreement, Fernandes also received two strikes and will have to register as a sex offender.
He will face six years and eight months in state prison when he is sentenced on March 28.
Fontana police began investigating the case in early February 2012 after the victim reported the crimes. That same month, Fernandes was put on administrative leave from the Rialto Police Department.
Investigators spent several months completing the case due to its complexity, police said.
Fernandes was arrested in May at the Fontana police station. Prosecutors originally charged him with 20 counts of sex crimes.
The remaining counts will be dismissed when Fernandes is sentenced, according to court records.
The 24-year-old victim, who was 14 when the crimes began, is a relative of Fernandes, officials said.
The crimes were committed between September 2002 and September 2006, according to a criminal complaint by the San Bernardino County District Attorney's Office.
Fernandes resigned from the Police Department in August. He had 17 years of law enforcement experience, police said, and was most recently a motorcycle officer assigned to the traffic division.
Reach Lori via email, call her at 909-483-9378, or find her on Twitter at @IEcourtsNow.