The politician (48) was the only candidate proposed to replace Adams as the deadline for other candidates to put their names forward expired yesterday evening.
A special Ard Fheis will be held on the 10th of February to ratify the change.
Earlier, Gerry Adams said that the current political atmosphere in Northern Ireland was "toxic".
"The truth is my freinds, I don't fill Gerry's shoes", she said, adding: " but I've brought my own and together we will walk a journey that will lead to a United Ireland". She called for Irish unity, describing it as "the best solution, the best collective arrangement".
"But I certainly never would have guessed that come February 10 2018 that I would be the boss of him", she said.
Mr Adams announced in November that he was stepping down as Sinn Fein president after 34 years in the role.
She added: "Some of you have said to me "you have very big shoes to fill".
Nicknamed "Marmite Mary" because she divides opinion sharply, Ms McDonald is the TD (member of the Irish Parliament) for Dublin Central and began her political life as a member of Fianna Fail, one of Ireland's two mainstream political parties.
Mr Adams also used the Belfast gathering to indicate Sinn Féin wants to see an end to the stalemate at Stormont.
"Uniting this island is the best outcome for all our citizens and it is now our task to convince our unionist friends and neighbours of that and to encourage them to help us build a new Ireland".
Sinn Fein is now the third biggest party in the Republic of Ireland and the second biggest party in Northern Ireland.
Many party members have been tweeting their support for Ms McDonald.