What is the election process like in Iran?
His toughest competitor is expected to be Ebrahim Raisi, a previously-little-known cleric who registered last week and has since gained wide popularity among Rouhani's opponents. He is also a member of Assembly of Experts, an all-cleric body that will rule on the successor for the Supreme Leader.
Raisi also registered on Friday for the vote.
Earlier on Wednesday, Iran's former president Mahmoud Ahmadinejad had registered for the upcoming Iranian presidential election.
In this photo released by the Iranian Presidency Office, President Hassan Rouhani, left, speaks as he is accompanied by the head of Iran's Atomic Energy Organization Ali Akbar Salehi on a visit to the Bushehr nuclear power plant.
"From now on, protecting the deal is one of the most important economic and political issues", he said.
He is considered a close ally of supreme leader Ayatollah Ali Khamenei, who past year appointed him to run the powerful charity-cum-business-empire Astan Qods Razavi (AQR), which oversees the Imam Reza shrine in the holy city of Mashhad.
Ardavan Amir-Aslani, a French-Iranian lawyer who advises European companies setting up in Iran, said he would be surprised if Rouhani does not win re-election despite the economic malaise.
He said his decision to run was meant to help former Vice President Hamid Baghaei, a close confidant.
Frequently described as a moderate, Rouhani has served as president of the republic since winning 2013 polls.
The hardline Guardian Council, which vets candidates before they stand, has prevented other influential figures from running in previous elections.
Rouhani, backed by Iranian reformers, has high chances to become a frontrunner in the presidential race.
The Guardian Council will assess the qualification of the applicants and announce the names of the qualified candidates by April 27.
Rouhani has on several occasions called for massive turnout in the elections slated for May 19. "There is extensive pressure on me from dear people of different walks of life as their small servant to come to the election".