Ali Akbar Hashemi Rafsanjani, who served as Iran's president from 1989 to 1997, has the lead in national opinion polls, but had wavered on whether to stand for election.
"Despite my prior doubts, today, based on necessity I again announce my readiness to enter the executive field and I put myself at your disposal," Rafsanjani said in statement.
Iran's current two-term president, cleric reformist cleric Mohammad Khatami, is barred from seeking another consecutive term in office.
Rafsanjani last ran for office in 2001 when he was defeated in a race for a seat in parliament.
Some 90 people have registered as candidates in the current presidential election, a number that will be narrowed to a handful of candidates by Iran's Guardian Council, a conservative group that has the power to disqualify any candidate. In 2001, the Council reduced the number of candidates from 800 to 10.
Rafsanjani threw his hat in the ring just a few weeks ahead of the June 17 election and during a period of growing tensions between Iran and Western countries that have called for a halt to Iranian nuclear development activities.
Political allies of Rafsanjani told news outlets the former president's candidacy had been opposed by hardliners in the Iranian government who are wary of his ties to the West.
On Tuesday, the government in Tehran said it would re-start uranium enrichment, a process that can produce fuel for power plants or components of nuclear weapons. The move is strongly opposed by the United States and the European Union, whose officials have said Iran's activities could result in sanctions being imposed by the U.N. Security Council. Iran has said all of its nuclear development is for civilian purposes.
Iranian government officials said the plan for new uranium enrichment stemmed from frustration over the slow pace of discussions with European negotiators over the future of Iran's nuclear programs.
In his statement, Rafsanjani said his goal was to "to remove tensions and build international confidence."