Financial terms were not disclosed.
The deal runs through the 2018 season and has team options for 2019 and 2020.
Doolittle, 27, is in his third season with the A's and in 122 appearances has 46 holds and four saves. The team drafted him as a first baseman in 2007 even though he won 22 games as pitcher in college, but he developed knee problems as a professional and switched to pitching.
This season in eight appearances, Doolittle has four holds and one save in 8.2 innings for the Athletics, who have started the season 10-5.