Darby joins Bradford on a free transfer after he was release by Liverpool earlier this summer.
The 23-year-old has signed a two-year contract with the Bantams.
“Bradford is obviously a big club and the ambition of the manager really sold the place to me,” Darby told the club’s official website.
“Once I’d spoken to him, that really made my mind up to join here. This is the first club I’ve signed for permanently away from Liverpool so it really feels like a new chapter in my career for me.”
City boss Phil Parkinson added: “Stephen is a good young defender with plenty of the right attributes to succeed with us. He’s at the right age where he can still grow and improve with us, but still has plenty of league experience under his belt as well.”
