In a greeting to be published in the Bild newspaper, the pope praised the annual event as a festival of faith and "brother- and sister-hood".
He said he felt especially privileged to attend the festival, which is being held in his homeland. The German native will be making his first foreign trip there as pope to attend the gathering and celebrate Mass.
Meanwhile, some protest groups are demanding the Vatican pay more attention to its younger followers — and change its stand on condoms.
The week-long World Youth Day festival opens tomorrow.