Ackman has taken the stake with Sachem Head Capital, another activist hedge fund run by Scott Ferguson, who previously worked for Ackman.
Pershing said it would disclose details of its stake, which includes ownership of 41.8 million shares as well as 995,602 notional shares of Zoetis, in a regulatory filing on Wednesday.
Sachem Head owns about 1.6 percent of Zoetis.
Ackman's $18 billion hedge fund is known for making large bets, and speculation that Pershing Square was in the market helped send Zoetis' stock price up 8.9 percent to $43.72.
A spokesman for Zoetis said the company received a call from Ackman indicating that his hedge fund has made an investment in the company. Zoetis declined to provide details of that conversation.
"We are preparing for our upcoming Investor Day on Tuesday, Nov. 18, and we look forward to sharing more details about the company’s business model and growth strategies with the investment community at that time," Zoetis spokesman Bill Price said.
Shares of Zoetis, which have risen 34 percent this year, closed at a year-high of $43.72 on the New York Stock Exchange.
(Reporting by Svea Herbst-Bayliss and Ransdell Pierson and Narottam Medhora in Bangalore; Editing by Andrew Hay, Dan Grebler and Cynthia Osterman)
