Interior Minister Ousmane Badjie said the man was arrested on Thursday in a motel on the outskirts of the capital, Banjul, on a tip-off.
The US State Department said on Friday that two men had been arrested in the Gambia in connection with a possible threat to the US ambassador, Jackson McDonald.
It was not clear whether the second man had been released.
US officials have travelled to the Gambia to help with the investigation.
Non African
Mr Badjie did not reveal the suspect's identity or nationality, but said he was not an African.
"Presently, the man is helping security agents in their investigations. I do not want to pre-empt the interrogation, so I cannot tell you his nationality," he said.
A US State Department official said that the United States took the arrests "very seriously".
"We thank the government of the Gambia for their excellent assistance to the embassy on security matters, and to the United States on the war on terrorism," the unidentified source told the Associated Press news agency.
The agency says that in November last year, Gambian media reported the arrests of four Middle Eastern men suspected by the government of links to the terror network, al-Qaeda.
The men were released in December after being questioned by US investigators, Gambian newspapers said.
Gambia, which won independence from Britain in 1965, is a predominantly Muslim nation of 1.5 million.
Three years ago, President Yahya Jammeh denied reports that he planned to introduce Islamic Sharia law.