Manager Darrell Clarke said of his new signing; “I was impressed with Kelle when he played against us last season for AFC Wimbledon and his signing gives us added competition for the goalkeeping position.
“With the change in the loan transfer window, we have to ensure that all of our recruitment has to take place before the end of this month as after that we won’t be able to make any more signings until January.
“I am grateful to the club’s owner, Wael Al Qadi, the directors, and to Derby County for facilitating this loan and look forward to welcoming Kelle to the club.”
The 24-year-old shot stopper began his career in The Netherlands, his home country, before coming across to England for trials with Birmingham City and Bristol City at the beginning of the 2013/14 campaign.
However he returned home after failing to gain a contract, only to return to these shores in November 2013 when he signed for Nuneaton Town, for whom he was to make nine league games that season.
His form attracted the attention of a number of top clubs and he spent time on trial at a number of clubs in the summer of 2014, most notable with Liverpool and Everton.
In January 2014 Roos signed a two and a half year deal with Derby County and was on the bench for every one of their games in 2014/15. He made his Rams debut in a League Cup tie against Reading in September 2014.
He joined Rotherham on loan at the start of last season, and then went out on loan again in February, to AFC Wimbledon.
He appeared at The Mem, for The Dons, in March of this year and his final game for the club was at Wembley in the League Two play off final, against Plymouth Argyle, in May.