At 25 years old, Cengiz Under has this physical status:
Ünder began his professional career at Altinordu, and later transferred to Başakşehir. In his breakout season with Başakşehir, Ünder established himself as one of the best players in the league, scoring 7 goals in 32 games in his debut season in the Süper Lig.
On 16 July 2017, he became a Roma player for a €13.4 million fee. He scored his first goal for Roma in a 1–0 win over Hellas Verona in the Serie A on 4 February 2018. In the same month, he marked his Champions League debut with a goal; the opener in a 1–2 loss to Shakhtar Donetsk in the first leg of the first round of knockout fixtures, becoming the youngest Turkish player ever to score in the Champions League.
On 20 September 2020, Ünder signed for Premier League club Leicester City on a season-long loan, with the option of a permanent transfer. He made his debut in a 3–0 defeat to West Ham United, coming on as a substitute. On 25 October, he made his first goal contribution for Leicester when he assisted Jamie Vardy's winning goal in a 1–0 away win over Arsenal, which ended Leicester's 47 year wait for a victory away at Arsenal. On 10 December, Ünder scored his first Leicester goal in a 2–0 home win over AEK Athens in the UEFA Europa League.
On 4 July 2021, he signed on loan for Ligue 1 club Marseille, with an option to be bought of €9 million.
Ünder has represented the U18 and U19 levels of the Turkey national youth football team. He most recently was called up to the Turkey national under-21 football team.
In November 2016 Ünder received his first call-up to the senior Turkey squad for the match against Kosovo. He scored his first international goal in his second cap for his country in a friendly victory against Moldova in March 2017.