Dr Narong Sahametapat, Permanent Secretary for Public Health, said that the Guinean woman had a high temperature three days after her arrival in Thailand.
She is being questioned and treated at a provincial hospital.
The woman arrived in Thailand from Guinea on Aug 20.
Passing inspection at immigration on arrival, she sought medical care yesterday after having a high temperature. She is now being treated in an isolation room.
The Department of Medical Sciences and Chulalongkorn University were assigned to check her blood samples and for the time being 16 Thai and foreigners are under close watch for possible EVD infection.
Dr Narong advised the public not to panic.
He said people could live their normal life while following up reports on EVD and its patients from the Ministry of Public Health.
Disease transmission was difficult and possible only through contacts with patients who developed high temperatures and other symptoms, he said.
The disease exists in patients' body fluids and vomit. Light touches would not result in infection, Dr Narong said.