Christa Rene Tharp, 33, of Napa, was arrested Friday at the Napa Police Department, Sgt. Chase Haag said.
Tharp coached softball at Napa High School and in the Napa Valley Express league for players under 18 years old, Haag said.
The alleged victim is between 14 and 18 years old, Haag said.
Tharp was arrested on suspicion of oral copulation with a person under 18, contacting a minor with the intent to commit a sexual offense, annoying or molesting a child under 18 years old, and sending harmful matter with intent to seduce a minor, Haag said.
She was booked into the Napa County Detention Center, posted bail, and was released. Her arraignment in Napa County Superior Court is scheduled for Sept. 2.
Police learned on July 11 about an allegedly inappropriate relationship between Tharp and the victim, whom Tharp met while coaching a local softball team, Haag said.
Police confirmed the relationship after interviewing Tharp and the alleged victim, Haag said.
The investigation is ongoing, and anyone with additional information is asked to call Napa police Detective Darlene Elia at (707) 257-9514.