[8] After scoring 15 goals and providing 7 assists in 14 league matches for Rosenborg, Tengstedt was sold to Portuguese club Benfica in January 2023, signing a contract until 2028.
[9][10] He made his debut for the club on 12 March, coming on as a substitute in extra-time of a 3–0 victory away at Marítimo, in the Primeira Liga.
[12] A month later, on 20 September, Tengstedt made his UEFA Champions League debut, coming on as a substitute for the final minutes of a 2–0 home loss to Red Bull Salzburg, in the competition's group stage.
[13] On 12 November, he scored the 97th minute winner in a 2–1 league victory at home to local rivals Sporting CP.
[14] On 29 November, Tengstedt made three assists, all to João Mário, in a 3–3 Champions League group stage draw at home to Inter Milan.