The North Seattle Boys and Girls Club says 28-year-old Augustine Williams was a councilor there. Police say Williams began a sexual relationship with a 17-year-old female volunteer last August.
Investigators say the two went on to have sex several times in the following months.
Williams has known the girl since she was in middle school and had acted as her mentor and supervisor at the Boys and Girls Club, according to police.
Club officials say they've investigating the incident.
"At the Boys and Girls Club of King County, we take the safety of youth as first priority. Any situation that challenges that safety is heartbreaking, and we take it very seriously," the club said in a written statement.