According to a Friday filing, the billionaire investor purchased 306,846 shares on Aug. 31 at $60.39 per share, valuing the trade at more than $18.5 million.
The stock gained slightly in extended trading on light volume.
Last Friday, the chairman of Icahn Enterprises said he bought another 2.3 million shares of the nutritional supplement company. As of the most recent filing, Icahn's total stake in Herbalife consists of more than 19.6 million shares.
Icahn and his long-time rival Bill Ackman have recently reignited a feud over the company. Earlier this week, Icahn rejected the idea that he is backed into a corner on Herbalife and insisted that Ackman is in a more risky situation because he has a short position in the stock.
Last week, Icahn said in a statement that he continues to believe in Herbalife and that "it's a great model that creates a great number of jobs for people."
— CNBC's Jeff Cox and Scott Wapner contributed to this report.