Apple closed at $469.25, down $6.93 or 5.4 percent on the day.
Icahn told CNBC on Wednesday he added to his already sizable position because the investment is a "no-brainer." The stock is relatively inexpensive and has upside potential.
"It's just extremely cheap," Icahn said on "Closing Bell," noting that by his calculation, Apple is inexpensive by the numbers.
If Apple's cash reserves are discounted, he said Apple has a market capitalization of around $300 billion, with earnings before interest, taxes, depreciation and amortization at roughly $50 million, generating a return of about 18 percent, or 5.6 times earnings.
Should Apple execute a share buyback of $150 billion, though, he calculates investors would only pay three times EBITDA for a 33 percent return or so.
Though Icahn admitted Apple's stock could continue to decline in the near term, he thinks it's likely to climb higher over time. He went so far as to list it as one of his finer investments, a bold statement for the hedge fund manager who also owns shares of such companies as Netflix, Dell and Chesapeake Energy, among others.
"I really think Apple is one of my best right now," Icahn said.