Laboratory tests confirmed on Wednesday that a 25 year-old woman was suffering from the disease, WHO spokewoman Fadela Chaib told journalists. Two more suspected cases in Baghdad were also being tested.
About 29,000 cases of acute watery diarrhoea have been recorded in Iraq over the past month, including 1,500 confirmed cases of cholera, mainly in the northeastern provinces of Suleimaniyah, Erbil and Kirkuk, according to the WHO.
Chaib said the spread of the bacterial disease into Baghdad was largely expected because of the intense movement of people and goods between the northern areas and the capital.
"For the time being we have only one case. It's likely that others will be suspected and identified," Chaib said.
"The most important thing for Baghdad, even if it's difficult, is to strengthen the surveillance system in order to be able to identify all the suspected cases," she added.
The WHO said it was ready to send more antibiotics and medical supplies if Iraqi health authorities requested.
Although cholera can easily be controlled through proper hygiene and treatment, the WHO's representative in Iraq warned last week that delays in ensuring access to safe water and food could lead to the further spread.
Ten people have died in Iraq since the outbreak was detected on August 23, Chaib said, underlining that the official death toll was largely unchanged compared to last week.
On September 14, the WHO reported about 14,000 cases of acute watery diarrhoea in northeastern Iraq.
Northern Iraq suffered an earlier cholera outbreak in 1999, while the disease struck the southern province of Basra shortly before the US-lead invasion of 2003.