Activist investor Trian Fund Management takes a stake in Comcast

Signage is displayed in the window of a Comcast Corp. Xfinity store in King Of Prussia, Pennsylvania.
Charles Mostoller | Bloomberg | Getty Images

Activist investor Trian Fund Management has taken a stake in Comcast in a bet that the media company's stock is undervalued, a regulatory filing on Monday showed.

The investor has taken a stake of 7.16 million shares, the filing said, adding that the change in holdings is as of June 30.

"We have recently begun what we believe are constructive discussions with Comcast's management team and look forward to continuing those discussions," Trian's spokesperson said on Monday.

Trian owns about 20 million shares in Comcast, for a roughly $870 million stake or about 0.4% of the total company, The Wall Street Journal first reported.

Comcast, parent of NBCUniversal and CNBC, did not immediately respond to Reuters' request for comment.