The Croatian had been a free agent since leaving Bolton in June, and he was linked with Deportivo La Coruna and Liverpool earlier this week.
However, it was Mainz who won the race for the player’s signature, announcing they had signed the player on a one-year deal.
Club’s general manager Christian Heidel admitted Klasnic was not their first choice target.
“He was always our plan B,” Heidel told club’s official website. “After the transfer of Adrian Ramos failed to materialise, we wanted to get this one wrapped up as quickly as possible.
“We were looking for an experienced player who knows the Bundesliga to go with our young attack. This is what we see in Ivan Klasnic. He has proven his quality, particularly his eye for goal, at all of his previous clubs.”
The Croatia international began his career at St Pauli, but it was at Werder Bremen that he made a name for himself.
Klasnic scored 49 goals in 128 Bundesliga games for Werder before joining Nantes on a free transfer in January 2009.
The Croat continued the good scoring form at Nantes, but left the club after they were relegated to the second division and has since been playing for Bolton Wanderers.