The tiny sliver of a country, named after the river from which British ships once allegedly fired cannonballs to fix its borders, joins the ranks of other Islamic Republics, including Iran, Pakistan and Afghanistan, Reuters reported.
"In line with the country's religious identity and values, I proclaim Gambia as an Islamic state," said Jammeh on state television on Friday.
"As Muslims are the majority in the country, the Gambia cannot afford to continue the colonial legacy," he added.
Gambia's population of 1.8 million people is 95 percent Muslim. He said that other citizens of other faiths would still be able to practice.