The Constitution stipulates that a minister's term is finished when he is convicted in court.
Sommai said at a press conference that he decided to resign as he respected the court's ruling and the Constitution.
The court found Sommai guilty of abusing power when as then permanent secretary to the Finance Ministry in 2004, he suspended from work Thatpong Wichuprapa, an executive of Thai Maritime Navigation Co, Ltd.
The company is a state enterprise under the Finance Ministry. Sommai ordered the suspension as chairman of the company's board chairman.
Thatpong brought the case to court.
Sommai told reporters that he had no intention to frame anybody when he decided to suspend the firm's executives.
Sommai, who was released on bail of Bt100,000, said he would appeal.