The world body received the letter from the tiny West African nation on Friday, said Farhan Haq, deputy spokesman for the UN secretary-general.
Gambia was one of three African countries that informed the UN chief last year they were withdrawing from the court that presses charges against alleged perpetrators of genocide, war crimes and crimes against humanity. The others were South Africa and Burundi.
Actual withdrawal from the ICC comes a year after notification.
Gambia's new president, Adama Barrow, has vowed to reverse other actions taken by his predecessor, Yahya Jammeh, who has been accused by rights groups of leading a government that tortured and killed opponents during his rule of more than 22 years.
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Many African nations have accused the ICC of unfairly targeting the continent. Last month, leaders from the 55-nation African Union adopted a non-binding strategy calling for a collective withdrawal from the court, saying it had focused too narrowly on prosecuting African leaders.
"As one of his first acts in office, President Barrow's notification to the UN secretary-general of The Gambia's decision to reverse withdrawal from the ICC is a crucial victory for victims of grave crimes and the rule of law," Clement Capo-Chichi, Africa regional coordinator with the Coalition for the International Criminal Court.
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