Mohamed Nasheed obtained 45% but needed more than 50% to avoid a run-off against his rival, Abdulla Yameen, who got 25% of the vote.
After decades of autocratic rule, the Maldives held its first free election in 2008, which was won by Mr Nasheed.
But he was ousted as president 18 months ago in an alleged coup.
Officials said the run-off was due to be held on 28 September.
Mr Nasheed's rival, Abdulla Yameen, is the half-brother of the Maldives' former President Maumoon Abdul Gayoom who led the country for 30 years.
Mr Nasheed resigned in February 2012 when army and police personnel joined opposition-led protests over the arrest of a senior judge.
The judge, Abdulla Mohamed, was detained in January 2012 after ordering the release of an opposition politician.
Mr Nasheed said he was stepping down to prevent "bloodshed", but later said he was forced to resign at gunpoint by police and army officers.
He was replaced by President Mohamed Waheed Hassan Manik, who had also been running in Saturday's election but polled only 5% of the vote.
Mr Waheed has consistently rejected claims of a coup by Nasheed supporters.
The leadership change sparked political unrest, leading to fears that the protests would have an impact on the islands' tourism industry.
UN Secretary General Ban Ki-moon said last week that he hoped the elections would be "credible and peaceful".
Key issues for voters included religion, nationalism, education and the economy.
Thoriq Hamid, a representative of poll monitoring group Transparency Maldives, said that the campaigning had been conducted "smoothly and peacefully".
However, there was still "some apprehension and confidence issues about the security forces", he said.
The presidential elections in the Maldives will go to a second round after frontrunner Mohamed Nasheed failed to win an outright majority.