He began his career with local side FC St. Pauli, and moved to Werder Bremen in 2001 after helping his previous team to promotion to the Bundesliga.

After a season in France with Nantes, he moved to English Premier League team Bolton Wanderers in 2009, initially on loan.

Born in Hamburg, West Germany, Klasnić started his professional career with local side FC St. Pauli, and impressed as a striker in the three and a half seasons that he played for the club in the 2.

Participating in the 2004–05 UEFA Champions League season, Klasnić scored five goals in the two group matches against Belgian side RSC Anderlecht, including a hat-trick in the 5–1 home victory for Werder Bremen, helping the team to advance to the first knock-out stage of the competition before losing to strong French side Lyon, who crushed them 10–2 on aggregate.

[7] Klasnić began the 2011–12 season with a place in the starting line up, and a goal, as Bolton beat Queens Park Rangers 4–0 on the opening day.

After losing his place in the team to fellow forward David Ngog, he came back as a late substitute at home to Queens Park Rangers on 10 March, where he scored the winner in the 86th minute as Bolton won the match 2–1.

By a quirk of fate, his international debut came against Germany in February 2004 in a friendly match where the Germans claimed a 2–1 victory.

In May 2006, he was expectedly named to the Croatia national 23-man squad for the 2006 FIFA World Cup finals in Germany and displayed really good shape throughout the month as he first scored four goals in three Bundesliga matches for Werder Bremen, ending the 2005–06 Bundesliga season with a total of 15 goals scored, and then netted a brace for the national team in a World Cup preparation friendly against Austria on 23 May, where they easily won 4–1.

In the 2006–07 season, he lost his place in Croatia's starting XI after giving two modest performances in the opening two qualifying matches for the UEFA Euro 2008, where he nevertheless managed to score one goal in the 7–0 crushing of minnows Andorra.

In the quarter-finals match against Turkey, Klasnić was again on the bench but was given the chance to play at the start of the first extra time in substitution of teammate Ivica Olić.

In the 118th (with just two minutes to play plus injury time), Klasnić headed an elevated pass provided by Luka Modrić and scored a crucial goal for Croatia.

However, Croatia was to be eliminated from the tournament as Semih Şentürk equalised with a goal just two minutes later and Turkey won the subsequent penalty shoot-out.

Ivan was remembered for his efforts which saw him become the only player to ever participate at a major tournament after a kidney transplant, also scoring two goals during his campaign.

However, in August 2007, the doctors at Werder Bremen forbade him to train due to inability of his body to withstand heavy physical strain.

Finally, on 24 November 2007, Klasnić was selected to start for Werder for the first time since December 2006, and played 65 minutes for his team in a Bundesliga match against Energie Cottbus.

He became the first player ever to participate at a major tournament after a kidney transplant when he played at Euro 2008,[14] and scored against both Poland in the group stage and against Turkey in the quarter-finals.