The UN agency's regional director for the Americas, Carlos Vogeler, predicted Wednesday that the milestone will be reached in the fall.
That figure would be about four per cent higher than last year's total.
Back in 1950, the figure was 25 million.
The UN counts only people who stay at least one night. It does not include cruise ship passengers.
Vogeler was speaking at a Caribbean Hotel and Tourism Association conference in Puerto Rico.
"It is quite iconic when you realize one billion people crossed borders," he said.
The WTO plans a celebration, and may even try to designate someone as the billionth tourist, when the number is hit.
The organization also projects that there will be 1.4 billion in 2020 and 1.8 billion in 2030.
The top three destination countries now are France, the United States and China.