Boss Steve Evans has been on the lookout for a new goalkeeper all summer following the release of Tony Thompson and the transfer-listing of Scott Loach and Roos fits the bill, joining the Millers on on a season-long loan.
The Dutchman, 23, was an ever-present on the Rams' bench during their Sky Bet Championship campaign last year, though played during the Capital One and FA Cup runs, making five appearances.
He will spend the 2015-16 campaign at the AESSEAL New York, but has penned a long-term contract with the Rams.
The Millers tried to bring Roos, who came through the ranks of Dutch Eredivisie giants PSV Eindhoven, into the club last season but were thwarted on several occasions.
Roos will join up with his new team-mates ahead of their pre-season tour to Scotland where he will begin the battle to earn the number one shirt ahead of the MK Dons game on August 8.
His arrival signals a likely end to the Millers' pursuit of last season's loanee Emi Martinez, who has been a summer-long target for Evans.
Martinez was hailed as one of the signings of the season following his arrival from Arsenal in March, with his performances helping the Millers claim their Championship safety.#rufc have signed @dcfcofficial duo Kelle Roos and Farrend Rawson on loan deals, subject to FL ratification.— Rotherham United (@OfficialRUFC) July 15, 2015
Roos is joined in South Yorkshire by a familiar face in Rawson, who spent an infamous spell with the club last season.
The 19-year-old played four times for the Millers, the last of which came against Brighton on Easter Monday - a game he later turned out to be ineligible for, resulting in a Football League points deduction.
Rawson will spend six months with the Millers.
“Since the end of last season I have made it clear that I
would like to strengthen the Millers in the goalkeeping department," said
boss Evans.
"For me there were only two options and I am delighted
that we have been able to secure one of them on a season-long loan. I have
watched Kelle on a number of occasions, as has our goalkeeper coach Andy
Dibble, both of us have on every occasion left the match very impressed.
“I am absolutely delighted to have secured the services of
young Faz. In a very short space of time towards the end of last season, he
joined us as a boy and left us as to go back to Derby County as a man."
The duo's arrival takes Rotherham's summer recruitment drive to 10.
