During the 2015–16 season with Altınordu, Söyüncü was linked with Beşiktaş,[5] Galatasaray and Sevilla, but he selected the Bundesliga because he thought it was better choice for his career and his improvement.

[6] On 24 May 2016, in the 2016 Bundesliga summer transfer window, Söyüncü joined German side SC Freiburg, which was promoted to the top flight competition of Germany for the 2016–17 season.

[14] Söyüncü had his Premier League debut on 27 October 2018 at the King Power Stadium where Leicester shared points with West Ham United following the final score of 1–1.

[15] Söyüncü scored his first Premier League goal in Leicester's 11th game of the 2019–20 season on 3 November 2019 against Crystal Palace at Selhurst Park, a 2–0 win.

[19] Four days later he played in the 2021 FA Cup final against Chelsea, a 1–0 win; Jacob Steinburg of The Guardian said he stood firm after fellow defender Jonny Evans was substituted with injury.

[26] In November 2015, he was called up to Turkey national football team by coach Fatih Terim, following the injury of central defender Serdar Aziz.

[37][38] Substituting for Ozan Tufan in the dying minutes of a friendly game against Sweden, Söyüncü earned his first senior national team cap on 24 March 2016.

[40] On 1 June 2018, Söyüncü scored his first senior international goal on a friendly game against Tunisia, held at Stade de Genève, Geneva, Switzerland.

[45] In 2016, he stated that his idol is Spanish defender Carles Puyol and he admires German international Mats Hummels.

[46] Former English international and current pundit Martin Keown analysed Söyüncü for BBC Sport in 2019, describing him "a defender made for the modern game", highlighting his agility, aggressiveness and heading attributes.