After the declaration of the Republic, he became a deputy. In this new era of his political career, he joined the founders of the opposition party, the Progressive Republican Party, and he was elected as the general secretary of the party in 1924. During the rebellion of Şeyh Sait, the Law on the Maintenance of Order was affected and the Progressive Republican Party was closed down. Ali Fuat Cebesoy was arrested with the accusations of participating in the attempt of assassination against Atatürk and was taken to İzmir. He was tried at the İzmir Independence Court and was acquitted in 1926.