Hamed al-Dafrawy said he was establishing the Peace and Development Society, a political front he said was open to all religious and societal groups.
"I decided to form the party because I disapprove of the (Muslim Brotherhood's) guidance bureau," he was quoted by Egyptian newspaper Al-Masry Al-Youm as saying.
Muslim Brotherhood Chairman Mohamed Badie said members of his reform movement were prohibited from acting outside the group's Justice and Freedom Party.
The Muslim Brotherhood was barred from competing openly in the Egyptian political system during the regime of Hosni Mubarak. The group, with candidates running as independents, took 20 percent of the seats in Parliament during 2005 elections but lost its seats to Mubarak's party in controversial elections last year.
The Muslim Brotherhood announced its decision to form a political party shortly after Mubarak's resignation in February. The reform movement said, however, that it wasn't fielding a candidate for president.
Egyptians during the weekend passed a series of constitutional amendments meant to usher in parliamentary and presidential elections this year. Critics are wary of the pace of political transition in the country.