The 23-year-old left-back, a free agent, has put pen to paper on a two-year deal and becomes Iron manager Alan Knill's third signing of the summer.
Kennedy told Scunthorpe's official website: "It's a good feeling and now I've signed I'm looking forward to the season starting. It's been in the pipeline for a couple of weeks, so we've not been waiting a massive amount of time, but it's good to just get it sorted.
"When I heard that Scunthorpe were interested I was as well. My agent let me know Scunthorpe were interested. I met the manager and he sold it to me basically.
"He told me about the football he wants to play and also about the aims and ambitions of the club and himself. It just felt right. That's the main thing for me. I need to get as many games as I can. I want to be playing football and that was a big factor in me coming."
Kennedy, who started out as a trainee at the County Ground, made 23 appearances for Paolo Di Canio's side last season, scoring one goal. He has also had loan spells at Gillingham and Rotherham.