Oil prices slipped Friday as the dollar surged after China introduced new stimulus measures to boost its flagging economy.

The benchmark U.S. crude contract ended down 78 cents, or 1.7%, at $44.60 a barrel on the New York Mercantile Exchange, settling back below $45 for the first time since Oct. 1. Prices have now lost 10% from their peak earlier in the month. The global Brent benchmark finished down 9 cents, or 0.2%, at $47.99 a barrel on the ICE Futures Europe exchange.

The selloff was prompted by a surge in the value of the U.S. dollar. The ICE Dollar Index has gained nearly 3% over the last seven sessions, its highest level since late August, against a basket of global currencies. Oil prices often move inversely to the dollar, as a strengthening currency makes it more expensive for investors to buy crude using foreign currencies.

The dollar’s gains came as China said it would cut interest rates by a quarter of a percentage point, in an effort to jump-start spending and economic growth. The move contributed to a darkening picture for the global economy, with Japan posting weak export numbers and Canada lowering its growth forecasts for 2016 and 2017.

China’s announcement also served as a confirmation of long-suspected slowdown in its economy, a bearish sign for oil as the nation is the world’s second-largest consumer after the U.S. and a key driver of demand growth. The backdrop for the oil market was already weak, with prices down more than 45% over the last year amid surging global production.

As the U.S. benchmark contract fell below $45 a barrel, it pushed out many traders who had bought at that level thinking the market was positioned for a rebound.

Bullish investors “are throwing in the towel again,” said Tariq Zahir, investment manager at Tyche Capital Advisors. “We’re still very well supplied. We’re back in the range we were in three weeks ago.”

Meanwhile, U.S. production remains robust, above 9 million barrels a day, despite a slowdown in output in recent months. Recent data showed domestic stockpiles rose by 8 million barrels last week, the largest increase since the spring, and stand near 80-year highs.

Bullish investors have been hoping to see further evidence of production cutbacks, but that has failed to materialize. The Baker Hughes drilling rig count, which tracks the number of rigs drilling for oil in the U.S., fell by just one on Friday from the previous week.

“Overall the fundamentals remain bearish, and the data suggests that the oil market will remain bearish for at least the medium term and likely beyond, well through 2016,” the Energy Management Institute said in a note.

In refined product markets, gasoline futures fell 0.2% to $1.3036 a gallon, while diesel futures lost 0.7% to close at $1.4544 a gallon.

Write to Christian Berthelsen at christian.berthelsen@wsj.com

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