Labour's Kim Leadbeater gave a thumbs up as she was officially sworn in as an MP at the House of Commons.
After swearing allegiance to the Queen, the new MP for Batley and Spen beamed a smile at fellow MPs, who could be heard saying "hear hear".
Ms Leadbeater, the sister of murdered MP Jo Cox, emerged victorious in the by-election fight last Thursday, when Labour hung on with a majority of 323 votes.
She was accompanied by party colleagues Shabana Mahmood and Dan Jarvis as she arrived in the chamber.
The 44-year-old later celebrated with fellow MPs, including Labour leader Keir Starmer and his deputy Angela Rayner.
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Speaking at a rally at New Palace Yard in Parliament, Ms Leadbeater said: "What an amazing team. I'm really clear that this election was won from every single one of you, every single door that was knocked on, every single conversation that we had with the amazing people of Batley and Spen and it was a huge team effort to get this result.
"But also, the work now begins. We've got work to do as a team, I've got work to do locally and nationally. But I am absolutely honoured and so incredibly proud to be part of your team so thank you so much for everything."
Mr Starmer welcomed his new MP, and said hers was "a victory of hope over division, decency over hatred".
He said: "You're the first new MP under my leadership. Everything you stand for is what our Labour Party stands for. Those values of integrity, of honesty and being of your community and for your community. "
"Those values that run through the Labour Party run through you and it is so good to see you here."
The new Labour MP was also hailed for her “bravery” by a minister, following a bitter and divisive campaign.
Defence Secretary Ben Wallace told the Commons: “Could I just congratulate the honourable member for Batley and Spen for taking her place and also say my personal admiration for both her bravery and sense of duty in putting herself forward to stand for that seat after the tragic loss of her sister.”
The Batley and Spen race, which was contested by George Galloway, turned nasty at points and saw Labour canvassers egged and assaulted, and Ms Leadbeater heckled in the street.
The result on Thursday will be a huge relief to Mr Starmer, however, amid rumours he would have faced a leadership challenge had Labour lost the constituency.
Writing in the Guardian, he called the vote a "turning point" for the party.
He said: “This by-election is a turning point – a sign that the politics of division and dishonesty won’t win out, that unity and decency can.
“And as we emerge from restrictions, there is now a real opportunity to harness the solidarity and national spirit that we’ve seen over the last year.
“To make sure that after the last year of sacrifice, we build a more united, fair, and secure country.
“To rebuild our economy and our public services, to bring our communities back together and to put honesty and integrity back at the heart of our politics.”
