The agency said "the outbreak seems to have been brought under control" in Syria after a push to immunise millions of children across the Middle East.
"It's a very encouraging sign that Syria's returning to a polio-free status," Chris Maher, the WHO's polio eradication manager, told AFP.
He said the landmark did not guarantee that Syria was polio-free, but it did suggest that the mass immunisation had been successful.
The WHO confirmed a polio outbreak in Syria in late 2013, reporting at least 10 cases of the disease in children hit with acute flaccid paralysis.
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To date, 27 million children have been immunised in eight countries -- Syria, Iraq, Lebanon, Jordan, Egypt, Turkey, Iran and the Palestinian territories, the WHO said.
Maher said between 2.7 and three million children in Syria had been immunised during the campaign in areas under rebel, government, and jihadist control.
"We were consistently reaching 85 per cent of kids or more in Syria in the last 12 months," he said.
The appearance of the disease in Syria, which had been polio-free since 1999, was blamed on the breakdown of the country's health infrastructure and vaccination programmes.
Polio has also affected Iraq, where immunisation efforts have been impeded in some places by fighting.
"But the proportion of children unreached by immunisation has been relatively low," Maher said, and no new polio cases have been reported in Iraq since April.