The caucus of Fa'atuatua i le Atua Samoa ua Tasi, or FAST, party was united in the decision to elect Fiame leader during a meeting on Tuesday.
Former HRPP MP and Speaker, Laauli Leuatea Polataivao Schmidt, is now deputy leader.
Fiame is the daughter of the first Prime Minister, the late Mata'afa Fiame Faumuina Mulinu'u II.
She is the second woman to take the leadership of a political party after one of her aunts, former MP and founding member of the HRPP party, Matatumua Maimoana Vermullen, founded and led the Samoa All People's party in the 80s.