Roos, 23, has joined on a season-long loan, but will be ineligible to play against Derby, having also agreed a new deal until 2018 with the Rams.
Rawson, 21, who spent a month on loan with the Millers last season, has signed on a six-month deal.
"Rawson's performances during his loan spell last season can only be described as outstanding," said boss Steve Evans.
"I repeat what I said last season - this lad will go on and play in the Premier League."
Rawson, who captained Derby's under-21 side, made four appearances for Rotherham last season.
Dutch-born Roos played five times for Derby after joining the club 18 months ago.
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Rotherham United have signed defender Farrend Rawson and goalkeeper Kelle Roos on loan from Derby County.