The 30-year-old has claimed that he is a better player than he was when he left Glasgow four years ago, after netting 128 goals in 192 first-team appearances and breaking the old SPL goalscoring record.
"It's a challenge to start scoring at the rate I scored before," Boyd is quoted as saying by The Express. "I want to continue where I left off.
"It might be four years on, but I'm only 30. I still have three or four years left in me. Last year I got myself fit over the summer, started scoring goals again and started enjoying my football.
"I'm sure, with the signings we've made and the fans we've got behind us, we can push on and hopefully win the Championship and get this club back to where it belongs."
Boyd's signing comes after Rangers secured the services of Kenny Miller and Darren McGregor, with ex-Hearts defender Ismael Bouzid on trial.