Rezai, 58, is a former chief commander of the IRGC. He ran in the 2009 presidential race but finished fourth, with less than 2 percent of the vote.
Rezai ran a campaign that blasted Ahmadinejad's mismanagement of the economy.
He is not considered a strong or charismatic candidate.
Rezai currently holds the position of secretary of the Expediency Council, which mediates between parliament and the Guardians Council.