The last official Iraqi census in 1997 put the population at 24 million, while the latest estimate in the CIA factbook, from July 2006, is around 27 million, the IOM said.
Rafiq Tschannen, IOM's chief of mission in Iraq, said: "The needs are enormous. Emergency supplies such as shelter and food are needed urgently for these people who are suffering both physically and psychologically."
"Those who are internally displaced are largely people who don't have the financial resources to leave the country," he added.
So far the IOM has received just $1 million, with pledges of a further $6 million, of the $50 million it has sought for its work in Iraq in 2007.
Last year it raised $9 million after appealing for $25 million.
In the last three weeks alone, almost 18,000 people have been displaced in the 15 central and southern governorates of Iraq, taking the overall number of displaced people from these regions to 290,000 since February 2006, the IOM said.
A further 84,000 people have been reported internally displaced in Iraq's three northern governorates over the same period, the group said.