He was part of their squad at the FIFA World Cup in 2018 and 2022, and the UEFA European Championship in 2020 and 2024, reaching the semi-finals of the 2020 tournament.

At first he was a defensive player and in Kenneth Zohore he had a teammate, with whom he was to play later in the Danish youth national teams.

He played his first match as a senior on 4 December 2011, when he came on the pitch against AC Horsens after 84 minutes, as a substitute for Mathias Tauber.

[5] His achievements attracted the interest of foreign and domestic clubs, and on 3 July 2013, he signed a contract with the newly promoted 3.

[12] Poulsen scored seven goals in 32 appearances during the 2015–16 season as RB Leipzig won promotion as runners-up to SC Freiburg.

[20] His team were runners-up in the 2019 DFB-Pokal Final; he scored in 3–1 wins in the first round at FC Viktoria Köln[21] and the semi-final at Hamburger SV.

[25] On 28 October 2021, he reached 300 matches in a 2–1 home loss to Club Brugge KV in the Champions League group stage.

[26] Poulsen was eligible to play for Tanzania but did not receive any offers from the Tanzanian federation; as a result, he chose to represent his native Denmark.

Poulsen was selected in Denmark's squad for the 2016 Summer Olympics, however he rejected the call-up in order to cement his place at Leipzig.

He scored his first goal in the European Championship in a 2–1 defeat to Belgium on 17 June,[33] adding another four days later in a 4–1 win over Russia.

RB Leipzig Individual In 2015 Poulsen founded a café in Copenhagen called Social with his friends Sebastian Streit Jeppesen and Frederik Nemo Andersen.