Rep. Matt Manweller has submitted a letter of resignation and will vacate his seat next month at the start of the legislative session.
The Ellensburg Republican, who had been pressured to step down amid a series of allegations of sexual misconduct, had previously said he would step down if he was re-elected. He received 61 percent of the vote in November.
The office of Gov. Jay Inslee received a copy of Manweller's resignation Wednesday, The Seattle Times reports.
In September, Republican leaders in the House called for Manweller to resign after The Northwest News Network reported that a former Idaho high school student of Manweller's said she had a sexual relationship with him in the 1990s, when she was 17.
Manweller, who was fired from his professor position at Central Washington University in August following an outside investigation into alleged inappropriate conduct, has denied the allegations.