The SNP’s Anum Qaisar-Javed affirmed her allegiance to the Queen, something required of all new MPs, as she appeared in the chamber at the start of business on Monday.
Ms Javed was last week elected to represent the constituency of Airdrie and Shotts.
The by-election had been sparked by former SNP MP Neil Gray’s decision to step down from the Commons and run for Holyrood.
Ms Javed, 28, retained the seat for the party – albeit with a majority down on the 5,201 held by Mr Gray in 2019.
She polled 10,129 votes, ahead of Labour candidate and local councillor Kenneth Stevenson who came in second with 8,372 votes, a majority of 1,757.
Ms Javed expressed her delight at being sworn in, adding in a statement: “I hope this moment is a source of encouragement for other young women and people from all minority groups to get involved in politics and to ensure Parliament is reflective of our diverse society.
“I will be getting straight to work on behalf of my constituents who have placed their trust in me and tackling the issues that matter to them.
“The Covid pandemic has exposed the deep inequalities that exist in our society and my priority will be working to ensure that my constituents have the support they need to get through this crisis.
“And, once the crisis is over, people in Scotland will have the right to determine our own future in a referendum. I will be campaigning for Scotland to become an independent country, so we can make the changes needed to secure a strong, fair and equal recovery.”