"The spread of cholera has slowed significantly in some areas compared to peak levels but the disease is still spreading fast in more recently affected districts, which are recording large numbers of cases," the WHO in Geneva said.
Nearly 2,000 people have died since the outbreak began at the end of April, the organization added.
Suspected cases of the deadly waterborne disease continue to infect an estimated 5,000 people a day.
Deteriorating hygiene and sanitation conditions, and disruptions to the water supply have exacerbated the spread of the water-borne illness.
Millions lack access to clean water, and refuse collection has ceased in many cities.
The Geneva-based International Committee of the Red Cross had expected the number of suspected cholera cases in Yemen to reach over 600,000 by the end of 2017.
Impoverished Yemen has remained in a state of civil war since 2014, when Houthi rebels overran much of the country, including the capital Sanaa.
The conflict escalated when Saudi Arabia and its Arab allies launched a massive air campaign in 2015 aimed at reversing Houthi military gains and shoring up Yemen’s embattled government.
According to the ICRC, over three million people have fled their homes since the start of the Yemen conflict, and more than 20 million throughout the country are in need of humanitarian assistance.