The 25-year-old, who can play in defence or midfield, played for Bradford on loan in 2007 and has now agreed a permanent deal with the club, to the delight of manager Phil Parkinson.
"It's an outstanding signing for us," Parkinson told the club's official website.
"Our recent Ireland tour really gave me a good chance to look at the balance of the side. I just felt we needed a good, versatile, utility-type player and Nathan Doyle was out there.
"Nathan can play all across the back four, as a holding midfield player and in other midfield positions as well. He's also played most of his career at Championship level."
Doyle started his career at Derby County and has also played for Hull City.