Watt, 25, returns to The Incora County Ground for a second stint, having taken seven T20 wickets in 2019, including best-figures of 4-19 against Yorkshire Vikings at Queen’s Park, Chesterfield, as Derbyshire Falcons reached Finals Day for the first time.
Since leaving at the end of that season, Watt’s international career has gone from strength to strength, culminating in a starring role for Scotland at the 2021 ICC Twenty20 World Cup, where he took seven wickets and maintained an economy rate of just 6.13 per-over.
The Scotsman has featured in 47 T20 matches for his country, claiming 56 wickets at an average of 20.78, with his best international figures of 5-27 coming against Netherlands in 2016, while he has claimed three wickets or more in an international T20 innings on seven occasions.
He will be available for all 14 Vitality Blast group fixtures, as well as any knockout matches, should Derbyshire progress.
Head of cricket, Mickey Arthur, said: “We have been looking for a left-arm spinner to mix up our bowling in T20 and Mark ticks all of the boxes for us.
“He will bring a different quality to our attack and, with Shan Masood and Suranga Lakmal also in the squad, we will have a good amount of international experience on the field at all times.
“Mark showed at the World Cup he has the ability to take the game to any opposition, he has done well in the Vitality Blast before for Derbyshire and I have no doubt he will be a key member of our team this year.”
Watt added: “I enjoyed my time with Derbyshire previously and the chance to work with Mickey was simply too good to turn down.
“I’ve worked hard on my game in the last couple of years, I’ve tested myself against some of the best players in the world and I’ve been able to show what I can do.
“I want to pick up where I left off in 2019 with Derbyshire and target the knockout stages again. I know all about the quality already within this team and with the overseas we have added, I think we will be a match for any side.”