The 19-year-old will join up with the Derbyshire squad ahead of the Specsavers County Championship match against Nottinghamshire at Trent Bridge, starting on Friday (2nd).
The South Africa-born pace bowler, who holds a British passport, comes highly recommended by Surrey director of cricket, Alec Stewart, and will provide a fast bowling option in the absences of Hardus Viljoen and Will Davis.
McKerr has represented South Africa at under-19 level, making eight appearances, with best figures of 2-23.
Director of cricket, Kim Barnett, said: “The captain, vice-Captain and coaching staff felt the squad needed the impetus of a quick bowler.
“Conor comes highly recommended and has represented South Africa at youth level, impressing with his natural pace. Running in at over six feet tall, he can provide that strike bowling option that we currently lack with the injuries to Hardus Viljoen and Will Davis.
“The deal will also give Conor some first team experience, and I would like to thank Surrey and Alec Stewart for allowing him to join us, and on a free deal.”