Around 2.5 million people are expected to attend the event in Krakow, including Pope Francis, from 25-31st July.
Premier has previously reported that Polish authorities had security concerns for World Youth Day, including poor evacuation routes and the presence of Poland's largest river - the Vistula - nearby.
US authorities have also issued warnings for the Euro 2016 football tournament in France starting this month and the Tour de France cycling race.
France is still under a state of emergency after Islamist terrorists murdered 130 and wounded many more in attacks across Paris last year.
They've said they haven't received any specific information of a threat which prompted their warnings, but that countries hosting the events should be on guard.
