Doyle, who enjoyed a highly successful loan spell with City during the 2006/07 season, signed a one year deal this Saturday at the Coral Windows Stadium on the eve of the Bantams' pre-season friendly with Bury.
The 25 year old, who can play in a number of positions along the back four and midfield, then joined in with City's warm up for the Shakers friendly before watching the game from the stands.
Doyle will wear squad number 24 for the Bantams this season.
City boss Phil Parkinson said: "I think it's an outstanding signing for us.
"Our recent Ireland tour really gave me and Steve (Parkin) a good chance to look at the balance of the side. We both just felt that we needed a good, versatile, utility-type player and Nathan Doyle was out there.
"We've been working hard recently to bring him in and I think he'll prove to be a great piece of the jigsaw this season.
"When you sign a player and he's going to improve your starting eleven, you know it's a good signing.
"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.
"We'll have to work on his fitness a bit because he hasn't had a pre-season, so he won't feature next week (at Notts County), but we now have a good two-week period to work with him to get him up to speed.
"We raced real competition to get this deal done over the line and David Baldwin (City's Director of Operations) and myself have worked really hard to get it done.
"I said to Nathan than he can come here and be a real hero.
"Any group of players who bring success to our supporters will be heroes, and Nathan can be a part of that. He can play a really big part this season for us because he's certainly capable of that."
Doyle made 28 appearances in total for the Bantams during his first spell with City after joining on loan from home town club Derby County in August 2006.
Along with playing for Derby, the versatile performer has also featured for Hull City and Barnsley during his career.