Thousands of police have been deployed as voters are choosing two-thirds of the senate and the chamber of deputies. A presidential poll is set for October.
President Michel Martelly, who is barred from running again by the constitution, has ruled by decree.
The opposition has accused Michel Martelly of abusing his powers.
Polling stations across Haiti are due to open at 06:00 local time and will close at 16:00.
Nearly six million eligible voters are choosing 119 deputies and 20 senators from more than 1,800 candidates registered from different political parties.
The run-off round will be held on October 25 – the same day as the presidential election.
Several people have been killed since campaigning for Sunday’s poll opened in July.
Haiti’s police are being helped by UN police and peacekeepers to ensure safety during the voting.
Haiti’s parliament was dissolved in January over its failure to hold elections, leaving Haiti without a functioning government.
Haiti is one of the poorest countries in the world, and is still struggling with the legacy of the devastating 2010 earthquake.
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