In a letter to the Electoral Commission, Adama Barrow said he was standing down as a member of the main opposition United Democratic Party (UDP) so that he could represent the broad coalition that nominated him last month.
"As the standard bearer for the opposition coalition... I wish to inform you that I shall be contesting as an independent candidate," wrote Barrow, who until October was a little-known businessman.
"In the light of the above, I do hereby tender my resignation as a member of the United Democratic Party."
Barrow is expected to formally submit his candidacy on Wednesday, while President Yahya Jammeh -- who is expected to run for a fifth term -- has until Thursday to present his own name.
Several of UDP's top officials are serving jail sentences for holding public protests over the death of a party member in custody in April, and Barrow is expected to win the majority of opposition support.
His declaration came the day after the electoral commission accepted the candidacy of former ruling party parliamentarian Mama Kandeh -- the only other contender.
Kandeh modestly declared his chances of winning were "90 percent as of now", though strongman Jammeh, in power for 22 years, is expected to be a shoo-in.
Defending himself against accusations of being used to divide opposition votes, Kandeh sought to reassure the public that "no political party or individual" had pushed him to run.
"I am not funded or put up by President Jammeh to contest against him," Kandeh added.
Kandeh was expelled from the ruling Alliance for Patriotic Re-Orientation and Construction party in April 2013 for reasons that remain obscure.
Jammeh and opposition leaders had been due Tuesday to sign a memorandum on ensuring a "violence-free" election, but it was delayed until further notice.