Powerful middle order hitter Klaasen has become a mainstay of the South African white ball side, with his record-breaking 174 from 83 balls against Australia in a One-Day International in September underlining his ability.
He played four tests between 2019 and 2023, managing 108 runs at an average of 13.00, but was set to return to the team this year according to Conrad, with current wicketkeeper Kyle Verreynne failing to impress.
"After a few sleepless nights wondering if I am making the right decision, I have decided to retire from red-ball cricket. It’s a difficult decision that I have made because it is by far my favourite format of the game," Klaasen said in a statement on Monday.
"But for now a new challenge awaits and I am looking forward to it."
The 32-year-old has not elaborated on what that challenge may be, but with confirmed stints coming up in the SA20, Indian Premier League, the Hundred and Major League Cricket in the United States, it appears he will feature prominently in the global leagues in the shortest format.
Cricket South Africa’s Director of Cricket Enoch Nkwe confirmed Klaasen was still part of their white ball plans.
"We look forward to seeing him excel in the white-ball arena and continue to make valuable contributions to South African cricket in the years ahead," he said.
South Africa recently lost opener Dean Elgar to test retirement, while others who have prematurely given up first class cricket to focus on the T20 circuit include Quinton de Kock, AB de Villiers, Faf du Plessis and David Miller.
