The IOM put the number at exactly 2,834,676 and said a wave of displacement caused by fighting in Ramadi, the capital of the western province of Anbar, was the cause of the latest rise.
The organisation said that 133,000 people left their homes when the Islamic State group attacked parts of Ramadi a month ago. More than 16,000 have since returned to the city centre.
IOM Iraq chief of mission Thomas Lothar Weiss said "the quantity of life-saving humanitarian aid available is insufficient."
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There were around 300,000 internally displaced persons in Iraq at the beginning of 2014.
Unrest broke out in Anbar early last year, forcing hundreds of thousands from their homes. The biggest wave of displacement occurred when IS launched a huge nationwide offensive on June 9.
The International Committee of the Red Cross on Monday appealed for an additional $38.5 million from donors to fund its emergency response in Iraq.
It "will bring the total funding it requires for 2015 to 122 million, making Iraq its second largest operations in the world, right after Syria," said the ICRC.
The situation is even worse in neighbouring Syria, where the internally displaced population stands at 7.6 million and the response even more critically under-funded.