Born in Ilion, a district in west Athens, Greece, Siatounis began his career at Panathinaikos' youth academy in 2011, staying there for eight years.

[4] After playing with the under-18s, in the 2020–21 season Siatounis helped Sampdoria reach the semi-finals with the Primavera (under-19) team,[3] scoring six goals and making seven assists.

[9] On 2 January 2023, Siatounis joined Serie C club Virtus Entella, with Monza holding a buy-back clause.

[10] Siatounis represented Greece at under-17 level between 2018 and 2019, helping his team qualify for the 2019 UEFA European Under-17 Championship, where he played three group-stage games.

[11] A left-footed player, Siatounis transitioned from a winger, to a full-back, to a central midfielder during his youth career at Panathinaikos.