U.S. crude ended the session lower after the Federal Reserve announced it would not raise interest rates, citing the need for further improvement in the labor market and evidence that inflation will rise toward the central bank's target.
Higher U.S. interest rates typically boost the U.S. currency, making dollar-denominated oil more expensive for importers holding other currencies. The dollar eased 0.3 percent ahead of the announcement, and was last down 0.8 percent.
The Fed has held interest rates near zero since December, 2008. It last raised its benchmark fed funds rate on June 29, 2006.
Read MoreFed leaves rates unchanged
U.S. light crude oil settled down 25 cents at $46.90 a barrel, little changed from its price prior to the Fed's announcement. It traded between $46.33 to $47.71.
North Sea Brent crude oil was down 50 cents at $49.25 a barrel shortly by 3:09 p.m. EDT, having traded as high as $50.14.
John Kilduff, founding partner at Again Capital, told CNBC the Fed's decision is a net positive for crude prices. "This argues for a weak dollar in the short term. That dollar weakness lends some strength to prices here temporarily."
The question, he said, is how much ground the dollar gives up and how much that supports crude prices.