TAIPEI—Taiwan President Tsai Ing-wen won re-election by a historic landslide on Saturday, a decisive result widely seen as a rebuke to Beijing’s efforts to gain control over the island democracy.

Having cast herself as a firm defender of Taiwan’s democracy against China’s authoritarian influence, Ms. Tsai collected nearly 8.2 million votes, the highest-ever tally for a presidential candidate since Taiwan first elected its president by popular vote in 1996, according to the Central Election Commission.

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