Iranian opposition leader Mir Hossein Mousavi, under house arrest since 2011, will vote for President Hassan Rohani in Iran's election on May 19, according to the opposition Kalameh website.
The website reported on May 16 that Mousavi has requested that security forces bring a mobile ballot box to his residence so that he can cast his vote.
Mousavi is a popular reformist who was put under house arrest for his role in the months-long Green Movement street demonstrations after the contested 2009 presidential election.
His endorsement of Rohani comes two days after two other top reformists -- opposition leader Mehdi Karrubi and reformist former President Mohammad Khatami -- endorsed Rohani.
Though they have been banned from political activity and have not been seen in public for years, the three reformists remain popular with crowds at Rohani campaign events, which have loudly chanted their names.
Over the weekend, an immense roar came from the crowd when their images appeared on the screen at a rally.
In an online video endorsing Rohani, Khatami said, "We have started on a path with Rohani and we have come half way... We have resolved some problems and bigger problems remain for us to resolve on this difficult path with him."