The organisation said officials gave Min Whee-Kang 24 hours to leave the country, and reports suggested she had left overland to neighbouring Senegal.
There was no immediate official explanation but correspondents say The Gambia has been increasingly criticised over its human rights record.
President Yahya Jammeh, who seized power in a bloodless coup 1994, tolerates little dissent.
He has frequently launched broadsides at all sorts of groups – at various times threatening to kill rights activists, gay people and "saboteurs".
Rights groups say journalists have been harassed and dozens of people have been arrested and unlawfully detained in recent years, amid constant rumours of coup plots.
AFP news agency reported that Gambian officials told Ms Win to leave last Friday.
Unicef’s spokeswoman in Gambia told AFP: "Unicef received a letter last week from the government stating that the country representative, Ms Min-Whee Kang, was no longer welcome in The Gambia."
Source: BBC