The Former Wolves midfielder put pen to paper on a 12-month contact just a few days ago.
"Coming here is what I've wanted," Rankine told the club's website. "There were a couple of concrete offers, but I said to myself if United came in I would sign.
"One deal was financially better than this one - but you cannot put a price on enjoyment. I know what I'm coming into here and I believe we have the chance to equal what we did last year.
"It's good that United have been able to keep last year's squad together and pick up a few new players over the summer."