A former University of Hawaii math professor pleaded guilty yesterday to sexually assaulting a young girl over a six-year period, beginning when she was 7 years old.

As part of a plea agreement for five years’ probation, David A. Stegenga, 64, pleaded guilty in Circuit Court to 12 counts of third-degree sexual assault committed from June 1999 to September 2005.

Stegenga was arrested in September 2009 on the UH-Manoa campus, where he had taught since 1980. Court documents characterized Stegenga as a neighbor and family friend of the victim.

Seven of the charges were originally for first-degree sexual assault. The prosecutor agreed to the reduced charges in exchange for Stegenga’s guilty pleas.

First-degree sexual assault is a Class A felony punishable by a mandatory 20-year prison term. Third-degree sexual assault is a Class C felony punishable by up to five years in prison.

Stegenga agreed to serve five years of probation, and Circuit Judge Colette Garibaldi agreed yesterday to abide by the terms of the plea agreement when Stegenga is sentenced in May.

The state obtained the approval of the girl, now 18, and her parents for the plea agreement, said Kory Young, deputy city prosecutor.

Stegenga retired at the end of 2010.