The upset win by Republican presidential nominee Donald Trump spurred a sharp decline in U.S. stock futures and a broad flight to safety around the globe, underscoring the trepidation among investors about the New York developer’s presidency.

S&P 500 futures slumped as much as 5% following news that Mr. Trump pulled ahead of Democratic nominee Hillary Clinton. CME Group doesn’t allow its stock-market futures contracts to drop more than 5%.

Japan’s...