After one season he transferred to Thun in July 2012[4] and made his professional debut on 2 September 2012 in the game against Servette FC.

During the winter break of the 2015–16 Super League, on 12 January, Basel announced that Steffen had signed a four-and-a-half-year contract with them.

[5] Steffen played his first team league debut for Basel on 7 February, coming in as a substitute in the 74th minute, in the home game in the St. Jakob-Park against Luzern.

[6] Under trainer Urs Fischer Steffen won the Swiss Super League championship with Basel at the end of their 2015–16 season.

[13] He scored the first Wolfsburg goal after the temporary cancellation of the Bundesliga because of the COVID-19 pandemic in Germany, against FC Augsburg with a header from the edge of the box.

[16] Following the tournament, he returned to the national team and was frequently chosen in the starting lineup for 2022 World Cup qualifying matches.

On 16 March 2023, he was included in Switzerland's squad for their initial matches of their upcoming UEFA Euro qualifying campaign.