At least eight others were wounded in the midday blast, which is said to have hit people waiting at the al-Rafidain bank to collect their salaries.
Security personnel were said to be among those killed in the blast, which was confirmed by the town mayor, police and hospital sources.
It is thought to be the first major attack in the town since October 2009.
A suicide bomber killed five people at a funeral in that attack.
In Thursday's bloodshed, there were reports that security personnel were among the dead, but it was unclear whether they were police or soldiers.
The manager of the local hospital told Reuters there were many critical cases among the injured, and it was feared the death toll would rise.
Anbar province, where Haditha is situated, was once a stronghold for Sunni militants following the 2003 invasion of Iraq by US-led forces.
It gained notoriety when US Marines were accused of killing 24 Iraqi civilians in 2005.
But violence dwindled after tribal leaders allied themselves with US forces.
A suicide bomber has targeted a bank in the northern Iraqi town of Haditha, killing at least nine people.