Developer Activision Blizzard hopes that the matches Twitch will stream will help make e-sports mainstream.
Overwatch is hugely popular, with 35 million players worldwide.
The fact the deal was struck so late could affect viewing numbers, said one analyst.
The first-person shooter features about two dozen characters in team-based battles set across a near-future Earth.
In July, Activision Blizzard announced an e-sports league for the game that it hopes could eventually prove more lucrative than the UK’s football Premier League.
The e-sports competition will feature teams from different cities around the world.
The two-year deal with Twitch gives the platform exclusive worldwide rights to all the matches, which will be streamed in English, Korean and French.
“Blizzard has cut it fine by announcing Twitch as its streaming partner for the Overwatch League just a day before the start of the competition in Los Angeles on 10 January,” said Piers Harding-Rolls, head of games research at analyst IHS Markit.
“There is no doubt that the delay in confirming the distribution deal will have impacted the negotiations by teams for sponsorships and will give Twitch very little time to build awareness before the first match, which suggests the opening matches may have relatively low viewer numbers.”
Activision Blizzard said the deal was great “for our growing global fan base”.
“Our fans love to engage with content on Twitch, and we wanted to drive significant viewership of the Overwatch League in its inaugural season and beyond,” said Armin Zerza, chief operating officer of Blizzard Entertainment.
Kevin Lin, chief operating officer of Twitch, said: “The Overwatch League is making a major impact on e-sports by reshaping the industry with city-based teams.
“Given Overwatch’s consistent reign as a top-viewed game by our community, we look forward to offering their pioneering style of league play to a large and passionate fan base that will be able to bond over not only their favourite plays, but hometown pride.”
The deal was a huge boost for Twitch, said Mr Harding-Rolls.
“This cements Twitch’s position as the strongest Western player in live e-sports video, and I consider the platform a natural home for the Overwatch League,” he said.
“The deal will help Twitch penetrate into markets such as Korea, where local e-sports streaming platforms and broadcasters are traditionally dominant.”
Missing from the deal is China, home to the biggest e-sports viewing audience, and Mr Harding-Rolls predicted there would be a separate announcement about media partnerships in this country.
The Overwatch League will run, at the Blizzard Arena in Los Angeles, until June, with play-offs and finals scheduled for July.