Mohammad Reza Aref, a former vice president, told reporters that his supporters and Rouhani allies have agreed to release a joint list of candidates in order to more effectively challenge hard-liners and conservatives, who remain split.
He announced the move at a press conference, saying the bloc would be called the "Alliance of Reformists and Government Supporters" and would include several female and young candidates. Around 60 per cent of Iran's population is under the age of 30.
The vote will largely be a referendum on Rouhani following last summer's landmark nuclear deal, which curbed Iran's atomic activities in exchange for the lifting of international sanctions. The historic deal is expected to boost reformists and moderates.
In the 2013 presidential election won by Rouhani, the conservative vote was split between several candidates. The conservatives say they have learned from that mistake and are seeking a grand coalition with hard-liners. However, they remain divided.