The 28-year-old left Carlisle earlier this month at the end of his contract and pens a deal that will keep him at New York Stadium until 2015.
Collin, who began his career at Newcastle, rejected a move to an SPL club in favour of the South Yorkshire club and he is excited about the potential at the Millers.
"There was a bit of interest from League One and the SPL but I've chosen to come here because it's a club on the up with a great set-up at the training ground and the stadium," he said.
"The location and the fanbase are excellent."
Millers boss Steve Evans, who added Richard Brindley to his squad earlier on Thursday, added: "A lot of the success they (Carlisle) had in recent years was down to the big man's form.
"We identified him in February as a potential target, and there was interest from two established League One clubs.
"He's 6ft 4in, kicks it well, he's strong and has a big presence and has a real pedigree of course being from Newcastle United."