Watch out, Apple: Carl Icahn is coming for you.
Activist investor Icahn said Tuesday that he has taken a "large" stake in Apple, and he is pushing the company to use its cash to reward shareholders.
In a pair of tweets, Icahn said Apple is "extremely undervalued." Icahn had warned followers to watch his Twitter feed earlier in the week.
Apple shares jumped 5% on the news to $489 per share -- a six-month high.
Icahn met with Apple CEO Tim Cook on Tuesday to push for a larger stock buyback, and Icahn said the two plan to talk again soon.
Apple issued a short, vague statement: "We appreciate the interest and investment of all our shareholders. Tim had a very positive conversation with Mr. Icahn today."
Apple did not comment on reveal the exact size of Icahn's stake, and no regulatory documents disclosing the purchase had yet been filed.
Icahn is only the latest investor to push Apple for a stock buyback. In February, David Einhorn of Greenlight Capital publicly slammed the company for "hoarding" billions in cash.
Three months later, Apple announced it will return $100 billion to shareholders over the next three years, through stock buybacks and a dividend. A mollified Einhorn promptly increased his Apple stake.
Icahn thinks Apple could still do better. As of the end of last quarter, Apple had an impressive $147 billion in cash on hand.
Meanwhile, Icahn has also been embroiled in a lawsuit against Dell to keep the company public.