The 23-year-old, who has taken 38 first-class wickets at 27.73, will be available for the remaining T20 group fixtures, as well as the County Championship matches against Notts and Essex.
McKerr will be returning to familiar territory, having played for Derbyshire in two first-class matches in 2017 and taking 14 wickets, including becoming the youngest player in the club’s history to claim ten wickets in a first-class match, when he took 10-141 against Northamptonshire.
Head of Cricket, Dave Houghton said: “We’ve been keen to add to our fast bowling options, and Conor was high on our list of targets.
“He’s young, quick and gives us another variation in our T20 attack, while we’re eager to see him replicate his 2017 form in the County Championship.
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“Games are coming thick and fast, and there’s a need to rotate. We’ve been unfortunate with injuries and Conor’s quality will give us greater depth.
McKerr added: “Even though it was brief, I really enjoyed my time with Derbyshire in 2017 and as soon as I heard there was a chance to come back on-loan, I was interested.
“I’m keen to play T20 cricket, it’s a format that really tests your nerve and skill as a bowler, but has huge rewards when you get it right.
“I can’t wait to perform in front of the supporters.”
While his white ball opportunities have been limited, he has taken eight List A wickets, with best figures of 3-56.