Jara, who missed the decisive penalty as his country exited the recent World Cup at the hands of hosts Brazil in the last 16, has spent the past five years in England, representing West Brom, Brighton and Hove Albion and Nottingham Forest.
However, after being released by Championship side Forest, the 28-year-old full-back will now continue his career in Germany's top flight.
He joins a Mainz side preparing to feature in the UEFA Europa League following their seventh-placed finish last term.
Coach Kasper Hjulmand, who succeeded Thomas Tuchel in May, said of Jara: "He's a versatile player and fits the bill perfectly.
"He's industrious, a good tackler, technically strong, good in the build-up and a real team player.
Jara's contract includes an option for a further year and he is excited by the opportunity that awaits him.
"Having played in England for many years I am now very much looking forward to the challenge that is the Bundesliga," he said.
"I can't wait to meet my new team-mates as well as get to know the Coface Arena and the Mainz fans."
