Burnett was 8-18 with a 4.59 ERA last season. The 37-year-old led the NL in losses.
Burnett's decision helps the rebuilding Phillies trim a hefty payroll. They spent more than $184 million on a roster that finished last in the NL East for the first time since 2000.
Burnett signed with the Phillies in February and earned $15 million in 2014. He is 155-150 with a 4.04 ERA in 16 seasons with five teams.