Santos Colon, 15 years old at the time, tried to recruit a sniper to shoot the pope at a mass service and planned to detonate explosives, the US Justice Department claimed.
The boy unknowingly recruited a covert FBI agent, and was arrested twelve days prior to ceremony in Philadelphia (pictured below) on 27th September 2015.
In a statement seen by AFP, the Justice Department said: "Colon engaged someone he believed would be the sniper, but in reality was an undercover FBI employee.
"Colon engaged in target reconnaissance with an FBI confidential source and instructed the source to purchase materials to make explosive devices."
Court files claim Colon, from Lindenwold, intended to undertake the attack in support of Islamic State, however, there no details concerning how or if he came into contact with the group.
By agreeing to plead guilty as an adult to one charge of providing material support to a terror group, three other charges filed against him as a juvenile were dropped.
Colon, who has previously been a mental unit patient, faces up to 15 years behind bars, however, sentencing may be postponed until 2021, while he undergoes psychiatric treatment.