The 28-year-old Scotland international becomes new boss Allan Johnston's first Killie signing since he left Queen of the South to take the reins at Rugby Park.
Barr - who struck the opening goal in Hearts' famous 5-1 Scottish Cup triumph over city rivals Hibs - was allowed to leave Tynecastle by the administration-hit club at the end of his contract.
He started his career with Falkirk and, after being named Bairns skipper at the age of just 22, he was rewarded for his club form with a call up to the Scotland squad in August 2008, making his debut against Northern Ireland in a goalless draw at Hampden.
A spokesman for Kilmarnock said: "Darren joins as a free agent following his departure from Hearts at the end of last season and brings a vast amount of experience to the club having made over 200 SPL appearances over the course of his career.
"Everyone at Kilmarnock Football Club would like to welcome Darren to Rugby Park and wish him every success during his time here."