European stocks ended lower Tuesday, reversing gains as oil prices sank toward bear-market territory, taking energy-related shares with them. The Stoxx Europe 600 Oil and Gas Index SXEP, -2.18% was shoved 2.2% lower, losing the most since November as West Texas Intermediate oil futures CLN7, -1.95% and Brent crude futures LCOQ7, -0.17% swung lower by about 3%.

U.S. stocks finished lower Tuesday as investors dumped energy shares after crude-oil prices sank into bear-market territory. The S&P 500 SPX, -0.67% dropped 16.43 points, or 0.7%, to close at 2,437.03, with nine of the 11 main sectors trading lower. The energy sector was down 1.3%, topping the losers.

Stocks were lower across the Asia-Pacific region early Wednesday, as global price declines for oil hurt energy companies, though mainland markets were resilient after MSCI Inc. said it would include Chinese stocks in its emerging-markets index. Oil prices returned to bear-market territory overnight and the U.S. benchmark has fallen 20% from its last high point, with cuts by the Organization of the Petroleum Exporting Countries offset by increasing production elsewhere.