The 31-year-old spent part of last season on loan at League One rivals Wycombe before being released by parent club Hull in the summer.
He will add experience to Dean Saunders' midfield, which has already been bolstered by the additions of David Cotterill and David Syers.
"It's good to have signed - I'm pleased to be here," Harper told the club's official website.
"When the gaffer and Brian Carey say they want you down here then you've half got your foot in the door already. There's a decent set of lads here, it's really a Championship team, and I know we got relegated, but there's a good team here.
"The gaffer said he wanted me down here, and when someone puts their faith in you it's nice. There were lots of clubs this summer that said they were not sure - when someone says that they want you, it gives you a little lift."
