ACS Sepsi

László Diószegi and Dávid Kertész decided to start a new football club in the summer of 2011. They picked the red and white colors to honor the football traditions of Sfântu Gheorghe and chose the name OSK as an acronym reminiscent of the defunct Olt Sport Klub. They wanted the name of the city to be also included in the club's name, so they chose the prefix Sepsi form Sepsiszentgyörgy, which is the Hungarian name of the city. In order to be able to enroll to Liga V they signed a collaboration agreement with Clubul Sportiv Școlar from Sfântu Gheorghe and formed a junior team.

In their debut season, they won the Liga V competition of Covasna County and were promoted to Liga IV. They finished second in their first Liga IV season, eight points behind local Viitorul Sfântu Gheorghe. Valentin Suciu—who would eventually guide them to Liga I—was appointed manager in 2013. At the end of the 2013–14 campaign, Sepsi OSK effortlessly won the Liga IV competition of Covasna County and qualified for the Liga III promotion play-off, where they faced the champion of Vrancea County, Selena Jariștea. Following a 1–1 draw after extra time, they won 6–5 on penalties, being subsequently promoted to the third division.

After a satisfying third place in the 2014–15 Liga III, promotion to Liga II was set as the objective for the next season. They won the 2015–16 Liga III and were promoted, which represented a notable performance for the team at the time. In the summer of 2016, Attila Hadnagy joined the team and became its captain. As he scored 28 goals in 31 games, the unexpected happened in the season which followed. They finished second in the 2016–17 Liga II and were promoted to Liga I, the top tier of the Romanian football league system.

As a last step in becoming fully professional in 2017, the club started a series of investments into infrastructure which included the creation of training grounds for its youth center and building a UEFA Category 4 stadium. In the years which followed the promotion, Sepsi OSK managed to stay in the Liga I, qualifying for the first time to the championship play-offs in the 2018–19 season.

In the summer of 2020, Sepsi reached the Cupa României final for the first time, but lost it 0–1 to FCSB at the Ilie Oană Stadium in Ploiești. On their route to the last game of the competition, Sepsi eliminated Ripensia Timișoara, Astra Giurgiu, Petrolul Ploiești and Politehnica Iași; in all but one of the five matches "the Székelys" scored at least three goals.

In the 2020–21 season, Sepsi entered the championship play-offs for the second time in their history. They finished fourth place and went on to defeat Viitorul Constanța 1–0 in the European play-offs, which meant they would take part for the first time in European competitions. Sepsi was drawn against Slovakian side Spartak Trnava in the second round of the UEFA Conference League, but were eliminated due to a 3–4 penalty shoot-out loss after two draws.

The club secured a European spot again at the end of the 2021–22 campaign by winning the Cupa României final—left midfielder Marius Ștefănescu scored a double to bring Sepsi a 2–1 win over Voluntari at the Stadionul Rapid-Giulești in Bucharest. The club went on to win the 2022 Supercupa României with the same score against league champions CFR Cluj.

On 24 May 2023, Sepsi claimed its second consecutive national cup after a 5–4 penalty shoot-out defeat of Universitatea Cluj at the Stadionul Municipal in Sibiu, with goalkeeper Roland Niczuly saving three penalty shots. The club also repeated its performance in the Supercupa României, winning the 2023 edition 1–0 against Farul Constanța.