MacDonald accepted the position today at a meeting held in Lower Sackville.
She takes on the job after former premier Darrell Dexter stepped down as leader last week.
MacDonald, who represents the riding of Halifax Needham, served as health minister and as the province's first female finance minister as part of Dexter's cabinet.
She leads a party that was reduced to seven seats in the Oct. 8 provincial election.
Party officials say the process to elect a new permanent leader will be announced at a later date.