The organization said in a release Friday it had received "credible reports" that the executions were carried out early that day. Two in the group were described as Senegalese nationals and three were executed for treason.
"The decision of the Gambian president, Yahya Jammeh, to execute nine prisoners after more than a quarter of a century without execution would be a giant leap backwards," said Paule Rigaud, Amnesty International's Africa deputy director.
Amnesty International had counted Gambia among the 22 members of the African Union it considers "abolitionist in practice." Another 16 of the 54 member states have abolished the death penalty.
Jammeh, in a broadcast to mark the Muslim feast of Eid al-Fitr this week, said all death sentences imposed by the courts would be carried out "to the letter."
There were 45 men and two women on death row in Gambia, including three men sentenced during 2012, Amnesty International said. Under Gambian law, murder and treason are the only capital crimes.
Gambia, a former British colony, became independent in 1965. The smallest country in Africa, it stretches about 200 miles along the Gambia River in West Africa and is no more than 30 miles wide except for its 50 miles of Atlantic coast.