The organization said that 12 more cases are still being investigated. Most of the tested patients have been babies and toddlers.
Around 95 percent of Syrian children were vaccinated against the disease before the country's civil war began in 2011. Now, the UN estimates some 500,000 children have not been immunized.
WHO spokesman Oliver Rosenbauer said the source of the virus likely came from one of the endemic areas and was brought by foreign groups fighting in the country.
"That's from where it would have always spread, and that's why it is so important to eradicate in those areas, because otherwise you are going to keep seeing polio in polio-free areas," he told the BBC.
Polio, a viral disease that affects nerves and can lead to paralysis, has been eradicated from most developed nations, but continues to be a problem in Nigeria, Pakistan and Afghanistan.