Turner Broadcasting was a surprise winner earlier this year in the bidding for rights to the UEFA Champions League, Europe’s top club soccer competition. But what the company plans do with the rights may be even more surprising: Launch a stand-alone sports streaming service next year that will broadcast the vast majority of the games.

It was reported in February that Turner had won the U.S. English-language rights to the Champions League and the second-tier Europa League beginning with the 2018-19 season, but the acquisition — and Turner’s plans for it — was announced formally Thursday.

Many of the most important Champions League games, including the semifinals and final, will still be broadcast on cable television. But starting next summer, more than 80 percent of the matches will only be available to fans who pay for the new subscription service — the latest move by a media organization to monetize unused parts of a multimillion-dollar sports rights deal.

Turner, which owns cable networks including TNT, TBS, CNN and Cartoon Network, and already owns some rights for games in Major League Baseball, the N.B.A. and the men’s N.C.A.A. basketball tournament, will build its new product around soccer, at least to start.