Ackman's $20 billion fund, Pershing Square Capital Management, disclosed a 7.5 percent stake in Mondelez late on Wednesday U.S. time. Illinois-based Mondelez is a heavyweight among global packaged food makers with brands like Ritz and Cadbury.
Pershing Square said its beneficial ownership of 120.3 million shares includes forward purchase contracts and call options. Ackman's stake is valued at about $5.5 billion based on Mondelez's closing price on the Nasdaq on Wednesday.
"We welcome Pershing Square as investors in our company," a Mondelez spokeswoman said. "We'll continue to focus on executing our strategy and on delivering value for all our shareholders."
However, a source told CNBC that Pershing Square was considering a takeover of the company. Further details were not immediately available.
The Wall Street Journal (WSJ), which revealed the deal, reported that Ackman was betting the firm would soon merge with another food giant. Should that transpire, it would create significant after-shocks for the U.S. food sector and mark the latest development in a wave of industry-wide consolidation, the WSJ wrote.
- Reuters contributed to this report.