Girvan and South Carrick councillor Peter Henderson will retire at the end of this month.
The current SNP group leader will resign from his role on June 30 due to "personal ill-health".
Mr Henderson was formerly the leader of South Ayrshire Council, before the Conservatives took control in last year's election.
A short statement read: "I can confirm I will be retiring on June 30 as a councillor. The reason is personal ill-health.
"I do not wish to elaborate and hope I have served my constituents and South Ayrshire to the best of my abilities in my time in office."
The retirement could trigger a by-election in Girvan and South Carrick.
South Ayrshire Council has been approached for comment.
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