In 2012, Charalampopoulos moved to the Greek 1st Division club Panathinaikos. He made his pro debut in the 1st-tier levelGreek League, during the 2012–13 season, on 26 November 2012, in a home game against Panelefsiniakos. Charalampopoulos made his debut in the European-wide top-tier level EuroLeague, during the 2013–14 season, in a road game against Lokomotiv Kuban, on 13 December2013. On June 25, 2016, Charalampopoulos extended his contact with Panathinaikos for eight years, through the 2023–24 season. He was loaned to the Greek club PAOK, for the 2017–18 season. As part of his loan deal to PAOK, his contract with Panathinaikos was also restructured, as he signed a new 4-year contract with them, lasting through the 2020–21 season. The new contract also contained NBA buyout options after each season. On August 20, 2018, Charalampopoulos and Panathinaikos agreed to prematurely terminate their contract.
On August 14, 2019, Charalampopoulos signed a four-year deal with EuroLeague side Olympiacos. On September 15, 2019, he was loaned to Ionikos Nikaias of the Greek Basket League for the 2019-2020 season. His loan was prematurely recalled on January 8, 2020, and Charalampopoulos rejoined Olympiacos for their EuroLeague campaign, after a rather successful stint with Ionikos.
With the junior national teams of Greece, Charalampopoulos played at the 2011, 2012, and 2013 editions of the FIBA Europe Under-16 Championship. He won the bronze medal at the 2013 Under-16 tournament. He also played with Greece at the 2013 TBF Under-16 World Cup in Sakarya, Turkey, where he helped Greece win the gold medal, and was voted to the All-Tournament Team. In the summer of 2014, Charalampopoulos played at the FIBA Europe Under-18 Championship and helped Greece reach the fourth place of the tournament. He finished the tournament being first in minutes per game, third in points per game and fifth in total rebounds, earning his place in the All-Tournament Team. In the summer of 2015, Charalampopoulos suffered a right ankle injury in a friendly game against the USA, and doctors initially said that it would be difficult for him to play at the 2015 FIBA Under-19 World cup. Despite the injury, Charalampopoulos was able to play at the tournament, and he scored 21 points against the US in the tournament's semifinal. Greece lost the semifinal game against the US, and Charalampopoulos was then injured once again in the third-place game, against Turkey, which Greece also lost, as they finished in fourth place. Charalampopoulos also played at the 2015 FIBA Europe Under-18 Championship. In a game against France, he had 13 points, 10 rebounds, and 11 assists, making him the first player since Lithuania's Martynas Andriuškevičius to record a triple double at the same tournament. Eventually, Greece beat Turkey in the tournament's final, by a score of 64–61, and won the gold medal. Charalampopoulos was honoured as the MVP of the tournament. During the tournament, Charalampopoulos averaged 16.6 points, 8.1 rebounds, and 5.2 assists per game. Charalampopoulos then played at the 2nd division level2016 FIBA Europe Under-20 Championship Division B, where he won a bronze medal, made the All-Tournament Team, and was named the MVP. He also played at the 2017 FIBA Europe Under-20 Championship, where he won the gold medal and was named MVP of the tournament.
