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The activist investor Nelson Peltz said on Wednesday that he would seek a merger of PepsiCo and Mondelez International, going public with his plans after months of speculation.

Mr. Peltz, whose Trian Fund Management has quietly amassed a stake of more than $2.7 billion in the two companies, urged Pepsi to merge with Mondelez and then spin off its beverage business, creating a new snack food giant that would combine the Frito-Lay brands with Cadbury, Oreo and Nabisco.

Investors appeared receptive to the plan, which Mr. Peltz announced on Wednesday at the Delivering Alpha conference in Manhattan, sponsored by CNBC and Institutional Investor. Pepsi’s stock was up about 0.5 percent in afternoon trading to above $84 a share, while shares of Mondelez, the snack business spun off by Kraft last year, rose more than 2.5 percent to above $30.

Mr. Peltz, who said he had been in discussions with Pepsi’s management, proposed that Pepsi acquire Mondelez in an all-stock transaction worth $35 to $38 a share. Then, he said, he would like to see the company pay a dividend worth 20 percent of the market value before spinning off the beverage business.

“Pepsi is at a crossroads at this point in time,” Mr. Peltz said. “They’ve got a cash business, and they’ve got a growth business.”

A spokesman for Pepsi said in a statement, “We have a strong growth strategy and structure in place, and our results to date and returns to our shareholders prove that we are a high-performing company and our strategy is working.”

“We are confident in our ability to deliver long-term shareholder value as an integrated food and beverage company,” he added.

The proposed merger is among the most prominent activist campaigns waged by Mr. Peltz, an investor with years of experience in the food and beverage industry. His firm disclosed in April that it owned stakes in Pepsi and Mondelez, fueling speculation about Mr. Peltz’s intentions.

A combined Pepsi-Mondelez would likely be the largest snack food company in the world, a sweet and salty empire with significant reach in emerging markets. Pepsi had a market value of $129.9 billion as of Tuesday’s close; Mondelez was valued at $53.3 billion.

A merger would present a fresh opportunity for Mondelez, whose shares have languished since the separation from Kraft in October. The company, which is based in Deerfield, Ill., has failed to meet its own target for revenue growth.

In a statement, Mondelez said it “regularly engages in meaningful conversations with its shareholders and looks forward to meeting with Trian to learn about their perspectives in more detail.”

For Pepsi, which is based in Purchase, N.Y., and derives most of its sales from North America, a deal would offer increased access to emerging markets. But spinning off the beverage business would eliminate an element of diversification that is a hallmark of the company.

Mr. Peltz argued on Wednesday that the beverage business, though “wonderful,” was a laggard.

“The carbonated soft drink business is just not growing,” Mr. Peltz said on Wednesday. “Tastes change, people change.”

Mr. Peltz acknowledged that the salty snack business was similarly vulnerable to changing tastes. But he said the combined company would include certain products, like dark chocolate, that were perceived as healthier and could have an offsetting effect.

Any combination would likely be complicated, given the two companies’ range of products and differing distribution models, analysts have said. In addition, a merger agreement could potentially raise antitrust concerns.

For Mr. Peltz, this is familiar turf. The investor began in 2007 a campaign to improve Kraft, gaining representation on the board. He later supported Kraft’s breakup into grocery and snack food companies.

The talk surrounding Mondelez has attracted other prominent investors. The hedge fund manager William A. Ackman, who runs Pershing Square Capital Management, recently disclosed a stake in the snack food company, as did Ralph V. Whitworth of Relational Investors.

One casualty of a merger might be the name Mondelez, which Kraft invented last year after receiving submissions from more than 1,000 employees around the world. The name combines “monde,” the Latin word for “world,” and “delez,” a made-up word meant to suggest “delicious.”

To Mr. Peltz, it suggests nothing of the kind.

“The name Mondelez I hate,” he said. “It sounds like a disease.”