Teranga FM and Hilltop Radio were closed on Sunday, said Emil Touray, head of the Gambia Press Union, on Monday.
Teranga's headquarters were shut down by a police officer and four members of the National Intelligence Agency, he said.
"It is a slap in the face of the country's democratic process," said Touray. "People will not have access to information in this critical period of our history."
A government spokesman said he could not confirm the closures.
It was not immediately clear why the two stations were targeted by Jammeh, under whose 22-year rule the media has come under regular attack, rights campaigners say.
Teranga FM, popular for its review of newspapers in the local wolof and mandinka languages, has been closed four times in recent years.
The station's managing director Alagie Ceesay was arrested in July, 2015 and charged with sedition. He was hospitalised twice in early 2016 while still in detention, Amnesty International said, and later fled to neighbouring Senegal.
Jammeh, who seized power in a 1994 coup, initially conceded defeat to opposition rival Adama Barrow but then called for fresh elections, drawing condemnation from local opponents and foreign powers.
The leader's refusal to step down has opened up the possibility of a military intervention by West African forces after the ECOWAS body said it was putting military forces on alert. Jammeh called that a "declaration of war".
Barrow's election victory was seen as a surprising triumph for democracy in The Gambia, which gained independence from Britain in 1965 but has since had only two presidents.
But the elation seen on the streets of Banjul in the days after Barrow's victory was quickly extinguished by Jammeh's defiant stance.
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