Reddit Inc., the discussion forum known for its “ask me anything” online town halls with celebrities and politicians, was valued at $3 billion after a new financing round that brought in Chinese internet giant Tencent Holdings Ltd. as an investor, according to a person familiar with the deal.

Reddit raised $300 million in the latest fundraising and plans to use the cash to turn its freewheeling platform into more of an advertising powerhouse. Tencent accounts for half of the fresh financing for the San Francisco-based company,...