More than 60 seats are up for grabs in the highly contested second round of voting. Both conservatives and reformists failed to secure a majority in February's elections.
On Friday, Iranians began voting in the second round of elections for 68 seats of the 290-seat parliament, state media reported.
Polling stations opened at 8 a.m. local time (0330 UTC) in a highly contested runoff to February's elections.
"The voters will elect 68 lawmakers in constituencies that candidates failed to get 25 percent of votes cast in the first round of the election," Iranian Interior Minister Abdolreza Rahmani Fazli said, the Reuters news agency reported citing state TV.
The vote comes after President Hassan Rouhani's allies won strong public backing in the first round of voting in late February. Reformists, including moderates and centrists, swept all 30 seats in the capital, Tehran, for a total of 95 across the country, while independents performed well, garnering roughly 20 percent of the contested seats.
The pro-Rouhani List of Hope and the conservatives, who gained 103 seats, failed to secure a majority.
Friday's vote is crucial to bolstering Rouhani's reformist policies, which witnessed a landmark deal with world powers in 2015, ending years of international sanctions in exchange for Iran curbing its nuclear program.
The results of Friday's runoff are expected to be announced on Sunday, according to the interior minister. The new parliament is scheduled to begin its first session on May 27.