Klasnic returns to Bolton
The 30-year-old Croatian, who has signed a two-year deal, enjoyed a successful loan spell at the Reebok Stadium last season, scoring eight goals in the Premier League.
Klasnic's arrival follows that of Marcos Alonso last week and the signings of Martin Petrov and Robbie Blake earlier in the summer.
Klasnic told the club's official website: "I'm really pleased to be here now that everything is done. I spoke to the manager - he wants to work with me and I want to work with him. It was an easy decision to come back to Bolton."
Klasnic was brought in by previous manager Gary Megson but impressed his successor Owen Coyle enough for the club to want him to stay. He returned to Nantes at the end of his loan but had his contract with the French club cancelled earlier this week.
Hamburg-born Klasnic began his career with German side St Pauli before going on to spend seven years at Werder Bremen. The forward, who joined Nantes two years ago, has won 38 caps for Croatia and scored 12 international goals.