"Tmrw we'll be sending an open letter to @tim_cook. Believe it will be interesting," he tweeted through his handle @Carl_C_Icahn.
The investor has in the past urged Apple to return more of its cash to shareholders and pressured the company to raise its stock buyback and dividend.
Icahn also tweeted that his fund had announced a large position in Apple just over a year earlier and noted that the company's stock had risen 50.6 percent since then.
Icahn was not immediately available for comment.
Apple shares were up 2 percent at $100.73 in late afternoon trading. More than 43 million shares had changed hands by 15:11 p.m. ET.
"This story is developing."
© Thomson Reuters 2014
Get your daily dose of tech news, reviews, and insights, in under 80 characters on Gadgets 360 Turbo. Connect with fellow tech lovers on our Forum. Follow us on X, Facebook, WhatsApp, Threads and Google News for instant updates. Catch all the action on our YouTube channel.