The Millers, recently relegated to League One, have announced their retained list, with experienced quintet of Shaun MacDonald, Billy Jones, Jamie Proctor, Kyle Vassell and Robertson all leaving.
Jones, Proctor and Vassell had been certainties to leave, having joined up on loan with Crewe, Wigan and Fleetwood during the winter window.
Robertson - who endured an injury-hit campaign at the AESSEAL New York Stadium, has been linked with a move to hometown club Aberdeen and is also likely to receive Championship interest.
Meanwhile, the club have opened discussions with right-back Matt Olosunde and goalkeeper Josh Vickers – whose deals are also set to expire shortly. – about the prospect of extending their stay in South Yorkshire.
Manager Paul Warne said: “It goes without saying that it is a sad time of the year when the team splits up, you never get the same dressing room the following year.
“The lads who are leaving have all played a part in previous seasons and I would like to place on record my thanks to each and every one of them for their efforts during their time with us.
“We ask a lot of our players here and they are always happy to step up and perform for our great club and our town. On behalf of the players, staff and supporters at Rotherham United I would like to sincerely thank each and every one of them and wish them nothing but success for the future.
“On behalf of everyone at Rotherham United would like to thank each and every player for their contributions to the club during their time at AESSEAL New York Stadium, and we wish them all the best for the future.”
Loan players Lewis Wing, George Hirst, Jamal Blackman, Ryan Giles and Florian Jozefzoon will all return to their parent clubs, following the conclusion of their loan spells with the Millers.