Less than a day after Snap Inc. posted weak results that sent its shares plunging, the struggling social-media and camera company disclosed that Chinese internet giant Tencent Holdings Ltd. had recently bought a 12% stake, becoming one of its largest shareholders.

Tencent, an early pre-IPO backer of Snap, acquired roughly 146 million of its shares in the public market, Snap said in a filing Wednesday. The purchase adds to an investment Tencent made in Snap in 2013 during a fundraising round before the company’s initial public...