Nicola Brogan has become the new Sinn Féin MLA for West Tyrone following Catherine Kelly's resignation last month.
Ms Brogan signed the register for MLAs this morning.
She said she was "looking forward to getting stuck in and standing up for workers and families in our area as we battle through these difficult times".
"My priority is working daily to improve people's lives and the communities in which we live," she said.
Deputy First Minister Michelle O'Neill said she was "delighted" to welcome Ms Brogan.
"Nicola will stand strong for West Tyrone, with a focus on housing, jobs and investment," she said.
"I have no doubt Nicola will work tirelessly and play a crucial role in building a new Ireland."
Northern Ireland's newest MLA was selected just weeks after Ms Kelly resigned at the end of October.
She became the fourth Sinn Féin official to resign over a failure to return money from a Stormont emergency Covid fund.
In a statement following her resignation, Ms Kelly apologised to her party, Sinn Féin voters and the wider public.
"I have been a signatory to that account for a number of years and as such had a responsibility to ensure it was operated to the highest standard," she said.
"That did not happen in this case."
