Michelle O'Neill has been named as Sinn Féin's successor to Martin McGuinness.
The 40-year-old health minister and MLA for Mid Ulster was announced on Monday as the party's new leader in the north.
Ms O'Neill described it as the biggest honour and privilege of her life.
She said being a republican was her "way of life" and told the party faithful at Parliament Buildings in Stormont: "I won't let you down."
Sinn Féin president Gerry Adams said Michelle O'Neill represented a "new generation" for the party.
"As a united all-Ireland team, we will give her the space and support to find her own voice and continue the good work Martin pioneered," he said.
Meanwhile, Martin McGuinness - who last week announced his exit from front-line politics in Northern Ireland after stepping down as deputy First Minister - also praised Ms O'Neill's work with the party.
Michelle O'Neill has represented Mid Ulster since 2007.
She served as agriculture minister from 2011 to 2016 before taking over in the health department last May.
In her acceptance speech for the role of northern leader, she said she plans to continue the "good work" of Mr McGuinness.
"I have never been afraid of a challenge and I have never been afraid to act," she added.