HONG KONG—Authorities barred democracy activist Joshua Wong from running in local elections scheduled for next month, a move that risks inflaming an opposition protest movement that has seen momentum in the streets wane in recent weeks.

In a letter dated Tuesday, Mr. Wong was informed that his candidacy in District Council elections had been declared invalid in accordance with the Basic Law, the city’s mini-constitution. In a separate statement, which didn’t name Mr. Wong, officials said support for Hong Kong self-determination...