The plunge in oil prices accelerated Monday in line with the selloff in global financial markets, with West Texas Intermediate crude futures settling under $39 a barrel as investors remain shaken about prospects for global economic growth.

WTI crude for delivery in October CLV25, fell $2.21, or 5.5%, to settle at $38.24 a barrel on the New York Mercantile Exchange. That was the lowest level for a most-active contract since February 2009.

Nymex prices had tapped a low of $37.75.

October Brent crude UK:LCOV5 on London’s ICE Futures exchange sank $2.77, or 6.1%, to $42.69 a barrel, dropping below $43 for the first time since March 2009. It is now trading nearly 59% below its one-year high of $103.19 reached in August last year.

These are “epic times in the oil market,” said Matt Smith, director of commodity research at ClipperData.

Still, “while the market does look weak from a fundamental perspective, sub-$40 [Nymex] oil means a lot of bad news is being priced in,” he said.

The oil losses deepened in the wake of losses in stock markets around the globe. In the U.S., the Dow Jones Industrial Average DJIA, -1.84% plunged 1,000 points at the open, then attempted to pare those steep losses.

Oil prices have been under pressure for several months due to oversupply concerns, but the slump has deepened in recent weeks on fears of a sharp slowdown in the Chinese economy and its impact on global markets and energy demand.

Read: China fears overdone? What investors need to watch

On Monday, the Shanghai Composite Index SHCOMP, -1.28% closed 8.5% lower, marking its worst one-day percentage performance since February 2007.

Read: How the market carnage is deepening, in four charts

“ ‘The doomsday scenario at the moment…is much weaker emerging-market growth worldwide.’ ” — Matt Parry, International Energy Agency

“From a pure demand perspective, the China risk is pretty much priced in,” said Matt Parry, senior oil analyst at the International Energy Agency in Paris, in an email. “The doomsday scenario at the moment, I suppose, is much weaker emerging-market growth worldwide.”

“The forward-looking balances, however, show signs of tightening, as relative demand growth in 2015-2016 outpaces forecast non-OPEC supply—the key wildcard being OPEC and Iran in 2016,” he said.

Read:5 reasons oil is below $40 a barrel

Back on Nymex, September reformulated gasoline US:RBU5 fell 7.4 cents, or 4.8%, to $1.471 a gallon and September heating oil lost about 7 cents, or 4.8%, to $1.393 a gallon.

September natural gas US:NGU15 ended at $2.65 per million British thermal units, down 2.6 cents, or 1%.

At the retail level, the average price for retail gasoline prices could fall below $2 a gallon, analysts said.

“The dive below $40 a barrel by WTI clears the way for a move to the low $30s a barrel, while Brent is trading below $45,” said Brian Milne, product manager at Schneider Electric. “Weak crude prices mean lower gasoline prices.”

Read: $2 gas? It’s already happening in 12 states

Sara Sjolin contributed to this article.