Polling stations opened around the conflict-scarred nation under tight security as the jihadists still pose a major security threat despite a sharp fall in violence.
The ballot comes with tensions surging between key players Iran and the United States over the nuclear deal, sparking fears of a destabilising power struggle over Iraq.
Roughly 24.5 million voters face a fragmented political landscape five months after IS were ousted, with the dominant Shiites split, the Kurds in disarray and Sunnis sidelined.
Prime Minister Haider al-Abadi — who took over as IS rampaged across Iraq in 2014 — is angling for a new term, claiming credit for defeating the jihadists and seeing off a Kurdish push for independence.
But competition from within his Shiite community, the majority group dominating Iraqi politics, should divide the vote and spell lengthy horse-trading to form any government.
Whoever emerges as premier will face the mammoth task of rebuilding a country left shattered by the battle against IS — with donors already pledging $30 billion.
Over 15 blood-sodden years since the US-led ouster of Saddam Hussein, disillusionment is widespread with the same old faces from an elite seen as mired in corruption and sectarianism.
More than two million people remain internally displaced and IS — which has threatened the polls — still had the capacity to launch deadly attacks.
Iraq has long been a crucible for the rivalry between Iran and the US, with Tehran exerting influence over Shiite politicians and Washington deploying troops to fight IS.
Overall, just under 7,000 candidates are standing and Iraq’s complex system means no single bloc should get anything near a majority in the 329-seat parliament.
Abadi — who has balanced off the US and Iran — is facing two leading challengers to his Victory Alliance.
Ex-premier Nuri al-Maliki is widely reviled for stirring sectarianism and losing territory to IS, but draws support from hardliners.
Hadi al-Ameri — a former transport minister — led Iran-backed paramilitary units that fought IS alongside Baghdad’s troops and heads a list of ex-combattants.
Votes in the Sunni heartlands once dominated by IS — including Iraq’s devastated second city Mosul — are up in the air as traditional alliances have been shredded by the fallout of jihadist rule.
Political forces in the Kurdish community — often seen as potential kingmakers — are also in disarray after a September vote for independence backfired spectacularly.
The Kurds look set to lose some of their clout on the national stage after Baghdad unleashed a battery of sanctions and seized back disputed oil-rich regions.
A senior security official told AFP that some 900,000 police and soldiers are on high alert to protect the vote, with airports and borders shut for the day.
Polling stations are open until 6:00 pm (1500 GMT) and initial results are expected in three days.