Iran will hold a second-round runoff on April 29 to determine the winners in 69 constituencies where no clear winner emerged in the February 26 parliamentary elections.
Ali Motlagh, secretary of the election headquarters, which is part of the Interior Ministry, made the announcement on state television on March 14.
Under Iran's electoral laws, a member of parliament must win 25 percent of votes cast, but this only happened for 221 of the country's 290 seats.
The first round on February 26 saw allies of Iran's relative moderate President Hassan Rohani make major gains against conservatives.
But neither group won a majority, meaning the outcome of the second round -- which will involve only the top two contenders from the first vote for each seat left empty -- could affect the balance of power in the legislature.