Born in
Steinfurt, Germany, and considered one of the most talented Finnish footballers of all time, Forssell made his
Veikkausliiga debut for
HJK at the age of just 16. who at the time were managed by
Gianluca Vialli. He made his debut as a substitute in a league game against
Arsenal in January 1999 and then three days later, Forssell made his first start for the club when he scored two goals against
Oxford United in the
FA Cup. His first league goal came three weeks later, when he scored against
Nottingham Forest. However, when Chelsea signed
Chris Sutton for £10 million, Forssell was loaned out twice to
Crystal Palace of the then First Division, scoring 16 league goals in just over 50 appearances. Forssell found it hard to find a place in the Chelsea first team with
Gianfranco Zola,
Jimmy Floyd Hasselbaink,
Tore Andre Flo and
Eiður Guðjohnsen, who were all ahead of him in the pecking order. he was sent out on loan again and had spells at
Borussia Mönchengladbach of the German
Bundesliga and twice to
Birmingham City of the Premier League. Forssell scored seven Bundesliga goals for Mönchengladbach in the spring of 2003 and then 17 Premier League goals for Birmingham in the
2003–04 season (more league goals than any other Chelsea player), during which he was the division's fifth highest goalscorer. He was named
Premier League Player of the Month for March 2004, a month in which he scored twice in a 4–1 win against
Leeds United, and won Birmingham's Player of the Year award. He returned to Birmingham on loan for the
2004–05 season, but suffered the second serious knee injury of his career. He returned to Chelsea to recuperate, and made occasional appearances for Chelsea that season, playing against
Bayern Munich in the Champions League and in the last home match, against
Charlton Athletic, as Chelsea celebrated winning their first top-flight title for 50 years.
Birmingham City In the summer of 2005, Forssell joined Birmingham on a three-year deal for a fee of £3.5 million after Chelsea decided that he did not fit in their long-term plans. He struggled in the
2005–06 season, not having found his best form after his injury, and could not save Birmingham from relegation. Forssell suffered another cruel injury blow to his other knee during training in October 2006. This left him on the sidelines until he returned to training in February 2007 after undergoing operations on both knees. He remained at Birmingham to have another go at resurrecting his career. His 2007–08 pre-season performances were good as he scored five goals in five games. His manager, Steve Bruce, was enthusiastic about his return to form. He continued his good form into the start of the season, scoring the opening goal of the match against Chelsea and consolidating his good form with two assists in the next three matches. He scored his first
hat-trick at club level on 1 March 2008, in the 4–1 win over
Tottenham Hotspur, a so-called "perfect hat-trick" – one goal scored with the left foot, one with the right, and one with the head. In his final season at Birmingham, he finished the season as their top scorer with nine goals as the club were relegated from the Premier League.
Hannover 96 In May 2008, following the club's relegation from Premier League, Forssell signed a pre-contract agreement to join
Hannover 96 on a free transfer when his Birmingham contract expired at the end of June. His contract with Hannover ran out at the end of the 2010–11 season. On his pre-season debut for Hannover, he scored 10 goals including an eight-minute hat-trick in one friendly game against amateurs FC Boffzen. The game ended up as a 23–0 win.
Leeds United After leaving Hannover when his contract expired at the end of the 2010–11 season, Forssell had trials with clubs including
West Ham United and
Leeds United of the English
Football League Championship (second tier). On 8 September, he returned from international duty with Finland to sign a contract with Leeds until the end of the 2011–12 season. Forssell was handed the number 18 shirt and took his place on the bench two days later against his former club
Crystal Palace. He came on as a second-half substitute to make his Leeds debut and assisted
Ross McCormack for their winning goal in a 3–2 victory. He came on as a substitute against fierce rivals
Manchester United in a 3–0 League Cup defeat. After mainly being used as a substitute and failing to score a single goal for Leeds, Forssell was told by
Neil Warnock that his one-year contract would not be renewed at the end of the 2011–12 season and that he would be released from the club at the expiry of his contract.
Return to HJK On 29 October 2012, it was announced that Forssell would be returning to Finland after spending 14 years abroad, signing a two-year contract with his former club HJK. On 22 April 2013, Forssell opened the scoring by a header, in a 2–0 away win over rivals
Honka. He finished the season with 14 league goals, the club's top goalscorer as HJK won their fifth consecutive championship. After a stint with German side
VfL Bochum, Forssell rejoined
HJK on 3 March 2016.
HIFK and retirement On 10 April 2017, Forssell signed a contract for one year with
HIFK. He announced his retirement in May 2018. ==International career==