The experienced striker was only a back-up option according to a senior official at the club, after the failure to capture their number one target
The 32-year-old has signed a one-year-deal with Die Nullfunfer, despite interest from Gaziantepspor in Turkey, following a two year stay at Bolton Wanderers.
"Klasnic has always been our plan B and after the Ramos transfer fell through we called Ivan on Saturday morning," he told reporters.
"We were looking for an experienced player who knows the Bundesliga to add to our young attacking department.
"We see that someone in Ivan Klasnic, who has shown his qualities, especially his goalscoring qualities, at every club he has been with so far."
Mainz will be hoping the 32-year-old can find his shooting boots with immediate effect after scoring just a single goal in their first two Bundesliga outings.