The 23-year-old goalkeeper penned a season-long loan from Championship rivals Derby on Wednesday after committing his long-term future to the Rams.
Despite an impressive reputation in the game Roos has only managed five first-team appearances for the Rams and spent the entire Championship campaign last year on the bench as Lee Grant's understudy.
He is now ready for games and is primed for the battle with Adam Collin to prove he should get the nod for the season-opener against MK Dons on August 8.
"Football is about battling so I am really excited to show what I have got and hopefully get a run in the team," he said.
"That’s what every player wants, I am excited to go for the challenge and show that I am the one that needs to play.
"It is always hard to have the patience to wait on your chance, every player probably struggles with that but I think I did well to keep my head up and this is the chance I am waiting for so I am excited to be here.
"This is really good for me, it’s the step I needed and I’m happy to be here and I can’t wait to show what I have got."
Roos came to the AESSEAL New York Stadium last season and sat on the bench for the thrilling 3-3 draw with Derby and he is looking forward to plying his trade their on a regular basis.
Roos' capture signals the end of a summer-long search for a goalkeeper by boss Steve Evans, who also tried to sign Emi Martinez following his impressive loan spell last season.
But with Arsenal committed to taking Martinez on their pre-season tour to the Far East, Evans turned his attention his other target and doesn't think he will be let down.
"The signing of Kelle once again endorses Rotherham United being right for the season ahead," the Millers boss said.
"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.
"The lad will now commence battle with Adam Collin to secure a starting place in the team.”
Roos' signing came on the same day as Scott Loach's exit was confirmed.
The former England Under-21 goalkeeper was surplus to requirements and had his contract cancelled, allowing him to move to Notts County.