Polling stations opened at 8:00 am for the ballot which is taking place in 21 provinces, but not Tehran, because no candidate in 68 constituencies managed to win 25 per cent of votes cast in initial voting on February 26.
Reformists who backed moderate President Hassan Rouhani made big gains in the first round following Iran's implementation of a nuclear deal with world powers, which lifted sanctions blamed for long hobbling the economy.
However Friday's voting -- in which the top two candidates from the first round contest seats head to head -- covers 55 smaller towns and cities where conservative support held up in February.
The split result in February -- reformists won 95 seats and the conservatives 103 with nominally independent candidates and minorities sharing others -- meant no faction won a majority.
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Iran's supreme leader Ayatollah Ali Khamenei urged strong turnout and has said that Friday's voting is no less important than the first round.
Mohammad Reza Aref, leader of the reformist pro-Rouhani List of Hope, has set a target of at least another 40 seats in the 290-seat parliament.
Gains for the president's allies would make legislative reforms more likely.
However if conservatives perform more strongly amid concern over the nuclear deal -- Iranian officials including Khamenei have complained that the United States is not honouring its commitments -- Rouhani's hopes for a more pro-government parliament could founder.
"We hope that people in this round can have a parliament in line with the goals of Imam and the leadership by electing principlists," said Gholam-Ali Hadad Adel, head of the conservative coalition.