Education
He then attended school and played basketball at Westtown, during the 2013-2014 academic school year and basketball season, as a high school junior.
He then attended school and played basketball at Westtown, during the 2013-2014 academic school year and basketball season, as a high school junior.
Born in Maroussi, Athens, Greece, he is a 2.18 m (7 ft 2 in) tall center. Papagiannis played at the Jordan Brand Classic International Game in 2013. He announced that he was committing to Westtown School in West Chester, Pennsylvania, in May 2013.
After his junior year of high school in the United States, he then transferred to the American School in Athens, Greece, for his senior year of high school.
Papagiannis made his senior club debut with Peristeri on January 5, 2012, at the age of 14, in a Greek Basket League game against Panathinaikos. He became the youngest player to ever play in the modern era of the Greek Basket League (since the league became organized by HEBA).
Panathinaikos On June 25, 2014, Papagiannis signed with Panathinaikos on a multi-year deal, yet there was a player option in the contract, which allowed him to leave Panathinaikos for a United States college during the summer of 2015. Papagiannis, considered to be one of the biggest young European talent prospects, received scholarship offers from 17 United States of America colleges, including UConn, Arizona, Temple, North Carolina State, Oregon, Saint John"s, and Kentucky.
On July 8, 2015, it was announced that Papagiannis had chosen to continue playing for Panathinaikos.
2012 International Basketball Federation Europe Under-16 Championship: Blocked Shots Leader, All-Tournament Team Club career.