At least 55 people have shown symptoms of the sometimes-fatal illness this week, including fever, lethargy, joint pain and severe headaches. Each complained of mosquito bites after visiting the popular park.
Japan's health ministry said domestic mosquitoes could have picked up the disease from someone infected in another country. There have thus far been no fatalities.
."Probably one person who was infected outside Japan went to (the) park. It's possible that there was an infected mosquito (transported to Japan) in a bag, but the patient numbers indicate that it was likely an infected person," said Takeshi Koruso of the Osaka University Research Institute for Microbal Diseases
He added the mosquitoes would not survive the colder weather of autumn in Tokyo.
The disease, for which there is no prevention protocol, struck Singapore in 2013, leading to four deaths and the infection of 12,000 people.
City officials have begun trapping mosquitoes in Yoyogi Park, and over 200 gallons of insecticide have been sprayed. Visitors are warned to wear long-sleeved clothes when visiting.