The billionaire investor purchased 1,832,402 shares on Thursday at $54.70 per share, valuing the buy at more than $100.2 million. The transaction increases Icahn's stake to 23.1 percent, from 20.8 percent.
The stock gained 1 percent in extended trading on light volume.
The purchase comes after Herbalife reported adjusted earnings of $1.21 per share on $1.12 billion in revenue on Tuesday. Analysts had expected earnings per share of $1.09 on revenue of $1.14 billion, according to a Thomson Reuters consensus estimate.
The company also announced on Tuesday that its CEO, Michael Johnson, will step down next year and will be succeeded by COO Richard Goudis.