About 800 people are thought to have been affected by the outbreak, at UNHCR's Hasansham U2 refugee camp, east of Mosul.
People began vomiting and complaining of stomach pains shortly after Iftar, the evening meal that Muslims eat to break their fasts during Ramadan.
Iraqi police have opened an investigation into what caused the outbreak at the camp, which houses refugees who have fled the ongoing battle for Mosul.
"UNHCR is extremely concerned by events at the camp, which has resulted in at least one casualty, with hundreds of other people falling ill," the UN's Refugee Agency said in a statement.
'Swift medical treatment'
"Staff have been working closely overnight to coordinate the response with other agencies and the relevant authorities in the Kurdistan Region of Iraq and Baghdad to ensure that those who have fallen ill were able to receive swift medical treatment and that the seriously sick were provided transport to nearby hospitals," it added.
UK-based charity Help the Needy Charitable Trust said about 2,000 meals had been distributed at the camp on Monday; it blamed a local restaurant for the food poisoning.
The UNHCR said additional health agencies have been brought in to assist with the response to the outbreak, and that refugees were being provided with extra clean water.
The Hasansham U2 refugee camp houses 6,235 people, many of whom left Mosul and the surrounding area to escape the fighting in their hometown.
Mosul has been the focus of an ongoing offensive
by Iraqi-led forces attempting to rid the city of ISIS fighters, who took control of the city in 2014.
According to the Iraqi government, more than 742,000 people
have fled the city to escape the fighting.
The camp was opened last month by UNHCR to accommodate the high levels of displaced civilians; UNHCR has opened or is building 13 camps near the city in response to the continuing crisis there.