[4][5] At the 2020 Adidas Next Generation Tournament Kaunas, he averaged 17.5 points, 5.8 rebounds, 3.8 assists, 3.8 steals and 1.5 blocks per game while Reyer Venezia finished in sixth place.

[8] That same year, he was named MVP of the U16 Torneo Villa de Íscar in Spain after scoring a game-high 15 points in the championship game win over the host team.

[15] Casarin averaged 8.8 points, 2.5 rebounds, 2.3 assists and 1.5 steals across four games at the 2019 FIBA U18 European Championship in Volos, Greece.

[1][2] He then played at the 2019 FIBA U16 European Championship on home soil, leading Italy to a bronze medal after averaging 15.5 points, 5.3 rebounds, 4.2 assists and 1.2 steals per game.

[2] In his senior national team debut on February 26, 2023, Casarin scored 10 points and got 2 steals in 11 minutes in a 72–68 victory over Spain in the 2023 FIBA Basketball World Cup qualification.

He was appointed president of Reyer Venezia by team owner Luigi Brugnaro in 2017, after previously holding the title of sporting director.