Ostler, who joined Tatler as editor in March 2009 from the London Evening Standard magazine ES, will be replaced by former Tatler fashion director Kate Reardon, currently contributing editor of Condé Nast sister title, Vanity Fair.
Nicolas Coleridge, managing director of Condé Nast, said: "Over the past two years, she [Ostler] has produced a widely admired and successful magazine, characterised by enthralling journalism, a string of memorable scoops and a rising circulation.
"The editions she has put together have been everything I hoped they might be when I appointed her. The 300th anniversary edition, the biggest in the magazine’s history, epitomised her wit and creative flair."
Reardon has been a contributing editor at Vanity Fair for 11 years. She has also previously worked as a columnist for The Times and the Daily Mail.
Reardon said: "After 11 enormously happy years at the incomparable Vanity Fair, I am excited to be joining the talented team at Tatler.
"I was made fashion editor of Tatler when I was 21 and it’s thrilling to return exactly 21 years later. Catherine Ostler has produced a wonderful magazine and I’m delighted to have been appointed editor."
Tatler recorded an ABC circulation of 86,448 for the six months to June 2010, up 1.6% year on year.