Poulsen was named 2001 Danish under-21 Player of the Year, and won the 20 Danish Player of the Year awards; the first to win two years in a row.Born in Asnæs, Poulsen started playing youth football in the local club Asnæs BK.Poulsen quickly made his mark on the league, and helped his club win the 2000–01 Superliga championship.
Poulsen was selected for the Danish squad at the 2004 European Championship.He played three of Denmark's four games, before Denmark was eliminated from the tournament.The transfer fee was €9.75M, with a four-year contract Poulsen made his league debut for Liverpool in a 1–0 win against West Bromwich Albion on 29 August 2010.He did not, however, endear himself to the Liverpool fans who often found his performances on the pitch lacking in skill.However, he moved to amateur club Holbæk at the age of 15, where he made his senior debut at 17 years of age, captaining the side on several occasions.
He was called up for the Danish under-19 football team in September 1998, and played four games for the team.
He has been a regular member of the Denmark national football team since his debut in 2001, and has played 92 matches, scoring six goals for his country.
He represented Denmark at the 2002 FIFA World Cup, 2010 FIFA World Cup and 2004 European Championship international tournaments.
On 20 November, he produced probably his best display in the centre of Liverpool's midfield against West Ham at Anfield, a game which the home side won 3–0.
Following the sacking of Roy Hodgson, and the appointment of new manager Kenny Dalglish, the winds changed not only for Liverpool but also for Poulsen.
He helped Sevilla defend the UEFA Cup title, winning the 2006–07 edition of the tournament and also won the Spanish Cup that year.