Federal officials said Nicholas Young, 36, sent an FBI informant about $250 worth of credits for mobile messaging accounts used by IS fighters.
Mr Young's arrest marks the first time a US police officer has been charged with helping IS militants.
Authorities said there was no threat to the Washington transport system.
Mr Young had been under investigation since 2010, the FBI said.
The FBI said that Mr Young had travelled in 2011 to Libya to fight with rebels trying to overthrow President Muammar Gaddafi. He packed body armour and other military gear, authorities said.
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