One of the oldest and largest investors in Tencent Holdings Ltd. is selling close to $10 billion of shares in the Chinese internet giant, reducing one of the world’s most lucrative tech bets at a time of turbulence for the sector.

Naspers Ltd., a South African media and internet firm, said Thursday it will sell 190 million shares of Tencent, cutting its stake in the company to 31.2% from 33.2%. The stock was priced at 405 Hong Kong dollars (US$52) a share on Friday morning in Hong Kong, a 7.8% discount to Tencent’s closing...