"It appears to be a single isolated case," the Ministry of Health said.
The case is Singapore's first in four months.
It is not yet known how the 27-year-old Singaporean laboratory technician contracted the illness. He is a postdoctoral student working on the West Nile virus, the ministry said in a statement.
Twenty-five people who came in contact with the man have been issued home quarantine orders, it said.
Acting Minister of Health Khaw Boon Wan told reporters that the man had no recent travel history and no known contact with any SARS patients.
Khaw said he thought the man posed a "low public health risk" because he was isolated quickly.
The case could mark the return of SARS, which killed more 900 people worldwide after it first emerged last November in China.
However, officials from the World Health Organization were hesitant to say the case was the start of a fresh epidemic.
By WHO's current definition of a probable case, there must be at least two SARS patients reported in the same hospital environment.
WHO regional spokesman Peter Cordingley said in the Philippines that Singapore officials have already traced people who were in contact with the man, "and none of them is sick."
Cordingley said the man had no lung infection or respiratory problems, which made his case unusual, adding that the WHO would like to see further testing to confirm the diagnosis.
Khaw also sought to play down the risk of another full-blown epidemic.
"I don't think this is a repeat of the crisis many months ago," Khaw said.
SARS virus samples were handled at the Environmental Health Institute lab at the National Environment Agency where the man worked, Khaw said.
But Khaw refused to speculate on whether the man may have gotten sick from samples in the lab.
The man also did research on the West Nile virus in a microbiology laboratory at the National University of Singapore. Both labs have been temporarily shut down as a precaution and staff have been given a leave of absence, the health ministry said.
Hitoshi Oshitani, WHO regional adviser for communicable disease surveillance and response, said earlier Tuesday that the man apparently did not specifically work on the SARS virus but that Singapore officials are checking whether he could have had contact with it in the laboratory.
The last case of SARS here was reported in early May and Singapore's tough isolation measures were praised by the WHO. The disease killed 33 and sickened 238 in Singapore.
More than 8,400 people were sickened worldwide before WHO declared in June that the disease had been "stopped dead in its tracks."
Singapore's SARS epidemic began in March this year and raged for three months, sending the city-state's economy into a tailspin. Visitor arrivals fell by as much as 75 percent and hotel occupancy rates plunged from their average 75 percent to as low as 25 percent as tourists and business travelers stayed away.
On Singapore's stock market, shares fell on news of the fresh SARS case. The index closed down 2.6 percent, or 42.14 points, at 1,580.14, its biggest one-day point loss since March 31.