Nelson Peltz, one of the world’s most dogged investors, has set his sights on Comcast Corp.

The billionaire’s investment firm, Trian Fund Management LP, said Monday that it had bought 7.2 million shares of the cable giant (CMCSA) through June 30.

Since then, the firm has amassed a total of about 20 million shares, according to a person familiar with the matter, who asked not to be identified because the full stake hasn’t been disclosed yet.

The holding represents a 0.4% stake in Comcast -- worth about $870 million -- amounting to a small but notable stake in a company whose market value tops $200 billion. And Trian is now bringing its concerns to Comcast’s management.

The news, previously reported by the Wall Street Journal, helped the shares rebound Monday from a broad market decline.

“Trian believes Comcast’s stock is undervalued,” the firm said in a statement. “We have recently begun what we believe are constructive discussions with Comcast’s management team and look forward to continuing those discussions.”

Comcast, which had fallen as much as 4.1% earlier in the session, closed down just 1.3%. The shares have slipped 3.6% in the past 12 months, compared with a nearly 10% gain for the S&P 500.

Peltz and his fellow co-founders at Trian -- Ed Garden and Peter May -- are known for pushing management to offload poor-performing businesses or even break up whole companies. They’ve taken on some of the world’s biggest corporations, including PepsiCo Inc., Procter & Gamble Co. and General Electric Co.

But they may have less power to influence Comcast, which was founded and built by the Roberts family. Brian Roberts, the chairman and chief executive officer, has a third of the voting rights, according to filings.

Comcast, based in Philadelphia, declined to comment.

Comcast’s broadband internet business has remained strong, even during the pandemic, but its media operations have had a tougher time. The company owns NBCUniversal and Sky, which have suffered from a broader media-industry slump, brought on by an advertising decline, a sports blackout, stalled production and shuttered movie theaters.

AT&T Standoff

Comcast isn’t the only media giant to come under the glare of investors. Last year, Elliott Management Corp. acquired a $3.2 billion stake in AT&T Inc. and began pushing for sweeping changes. But that campaign lasted less than two months. As part of peace deal, AT&T agreed to add two board seats and split the roles of chairman and chief executive officer. Seagate Technology Chairman Stephen Luczo was brought on to AT&T’s board as part of the accord.

Trian, founded in 2005, said last month that it had built positions in three new undisclosed companies. The investment firm said it had omitted confidential information in its quarterly filings, and that it would file separately with the U.S. Securities and Exchange Commission.

Investment firms are able to temporarily conceal new positions in companies in their quarterly disclosures with permission from regulators if, for example, they’re still building their stakes and fear that disclosing them would cause a stock to rise.

The confidential treatment on Comcast expired Sept. 21, according to Monday’s filing.