The US Justice Department said Santos Colon, 17, sought to recruit a sniper to shoot the pope as well as set off explosives as he performed mass in Philadelphia on September 27, 2015 at the end of the World Meeting of Families.
But Colon unwittingly recruited an undercover FBI agent for the job, and was arrested quietly 12 days before the event.
Colon pleaded guilty as an adult to one count of attempting to provide material support to terror.
But there were no other details on how he had become interested in the group and how he communicated with them.
The charge carries a maximum 15 years in prison.