The 28-year-old’s switch from the New York Stadium was ratified yesterday by the Football League, hours before Bantams head coach Michael Collins was sacked.
Paperwork had been lodged just minutes before last Friday’s 5pm deadline for the closing of the window, allowing Ball to become Bradford’s 17th signing of the summer.
“There were a number of clubs interested in me,” said the Millers forward, who was in the starting XI as Paul Warne’s men won promotion by beating Shrewsbury Town at Wembley last May.
“As soon as I heard Bradford had made an approach, I wanted to make it happen. I have played here on numerous occasions and am looking forward to doing so as a Bradford player.
“I will bring goals, I will bring assists and I will do all I can to help the club challenge.”
Bradford sit 17th in League One with six points from as many games. Ball added: “I have done it over the past few seasons in League One and I have been a part of teams who have played play-off football.
“In December last season, Rotherham were close to the bottom of the table and we turned it around and earned promotion. I know the division very well and I hope to add to the great deal of quality we already have here.”