After more than a decade apart, CBS and Viacom are reuniting.

In a deal that closed on Wednesday, the CBS Corporation, the company behind the broadcast network CBS and the publisher Simon & Schuster, merged with Viacom, the owner of Paramount Pictures and the cable outlets MTV, Nickelodeon and Comedy Central. The combination brings together a set of businesses that once dominated the media and entertainment industries but are now fighting to stay relevant in an increasingly digital world.

CBS and Viacom were already corporate siblings before the deal, both controlled by National Amusements, a theater company that grew into a major conglomerate under the mogul Sumner M. Redstone, who is ailing at age 96. His daughter, Shari Redstone, emerged as the company’s leader in recent years and had sought a merger since 2016. With the deal, Ms. Redstone cements her role as a trailblazing figure in a male-dominated industry, a woman whose peers now include leaders of media behemoths like Brian Roberts of Comcast and Robert A. Iger of the Walt Disney Company.

Now that Disney has joined Netflix and Amazon as a force in the streaming industry, the merger is meant to make ViacomCBS, as the new company will be called, a bigger player in digital entertainment than the two companies had been as separate entities.

Unlike its streaming rivals — a group that includes Apple, with its new Apple TV Plus streaming service, and AT&T, the owner of HBO Max, scheduled to make its debut in May — ViacomCBS will focus on supplying films and television series to other companies. With an executive team led by Ms. Redstone, the chairwoman of the company’s board, ViacomCBS intends to follow a strategy of selling its wares to the highest bidder as demand for original content increases.