The 28-year-old, who also played for West Brom and Brighton in England, could extend his stay with the Bundesliga club based on performances.
"Gonzalo Jara played himself into the limelight with some strong performances at the World Cup," said Mainz's general manager Christian Heidel.
"He's a very experienced, versatile player who will give our defence additional stability.
"We're delighted that he's chosen Mainz for his future."
Jara, who has been capped 69 times by his country, said he was "looking forward to the new challenge in the Bundesliga after many years in England."
He added that he was especially eager to meet his new team-mates and see the Coface Arena.
Jara's first task will be to help Mainz reach the Europa League group stages. They will discover their third qualifying round opponents in a draw held in Nyon this Friday.