Fleming, who has been with the team for two seasons, has decided to leave ahead of the 2025 season of The Hundred to spend more time with his family.
He was appointed in December 2023 and led the team to The Hundred final last season, where they were defeated by Oval Invincibles.
Fleming said: “I absolutely loved my time at Southern Brave, working in The Hundred and working with a great group of people at Utilita Bowl.
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“We got close to winning a couple of times so it’s difficult to step down but for family reasons I need to be able to dedicate more time to being at home this season.
“I wish everyone at Southern Brave the best for 2025.”
Giles White, Southern Brave director of cricket, said: “Stephen created an excellent environment for the players and it has been an enjoyable and successful stint with us.
“The team reached the eliminator and final during the two summers and we would like to thank him for his hard work and commitment.
“In his place we are delighted to welcome Adi Birrell and look forward to seeing him build on the team’s success in 2025.”
Birrell is the Hampshire Cricket men’s first team manager and Sunrisers Eastern Cape head coach.