Assistant coach Greg Blewett will lead the team in its last Marsh Sheffield Shield game of the season against Queensland at Karen Rolton Oval, starting next Tuesday.
Bradshaw thanked Siddons for his five years of service as Redbacks coach.
“Unfortunately the Redbacks have not been performing to our expectations in recent seasons,” he said.
“We acknowledge that the Redbacks played in consecutive Sheffield Shield finals in Jamie’s first two seasons as head coach, and also played in One-Day Cup finals in 2015-16 and 2017-18.
“I also know that Jamie is very proud that South Australia has had several players represent Australia across all three formats in recent months.
“On behalf of SACA, the Board, management and staff I would like to thank Jamie for his efforts as coach and, over a longer period, his outstanding service to South Australian cricket.”