Khatami, president from 1997-2005, said he would support former nuclear negotiator Rohani a day after reformist candidate Mohamad Reza Aref withdrew from the race, Iran's state-funded broadcaster Press TV reports.
Aref is the second candidate to pull out of the contest. He said he consulted with Khatami before abandoning the race, adding the end to his bid would give reformers a better chance on Election Day.
Rohani is the sole reformer left in the field of six candidates. The 12-member Guardian Council said Monday it wasn't targeting Rohani or his counterparts, though the clerical body can re-examine candidate qualifications before the vote.
Iranians head to the voting booth Friday to choose between the six remaining candidates in the race. Former Speaker of Parliament Gholam Ali Haddad Adel announced Monday he was pulling out of the race. He's from the Principlist camp, conservative lawmakers allied with Iranian Supreme Leader Ali Khamenei.
Khamenei hasn't expressed support for any single candidate. He said a high voter turnout would send a strong message about the strength of Iran to its adversaries.