Fiame Naomi Mata’afa was sworn in during a makeshift ceremony held in a tent on the statehouse lawn in the capital, Apia.
Her party, known by the acronym FAST, narrowly won the 9 April general election, which sparked a month of legal challenges and calls for a second round of voting.
UN stands ready
“He urges the leaders in Samoa to find solutions to the current political situation through dialogue in the best interest of the people and institutions of Samoa”, it said.
“The United Nations stands ready to provide support to Samoa if requested by the parties.”
Samoa is a Pacific island nation of approximately 200,000 people and this was the closest-run election in its history.
Changes and challenges
Ms Mata’afa, 64, is a former Deputy Prime Minister and will join Jacinta Arden of New Zealand as one of the only women leaders in the region.
The FAST party was founded last June, challenging the ruling Human Rights Protection Party (HRPP) which has been in power for nearly 40 years.
HRPP is headed by Tuilaepa Sailele Malielegaoi, who has served as Prime Minister since 1998.
Both parties won 25 seats each in the election, but the lone independent parliamentarian sided with FAST.
Despite a challenge by the HRPP, the Supreme Court upheld the result and ordered the swearing-in ceremony to take place.
However, over the weekend, Samoa’s Head of State, Tuimalealiifano Vaaletoa Sualauvi II, suspended the parliamentary hearing on Monday for the ceremony.
The Supreme Court subsequently ruled that it should go ahead, but FAST members and supporters found the Parliament doors locked when they arrived on Monday morning.