Ms Leadbeater was welcomed to the House of Commons for the first time on Monday afternoon by a large number of Labour MPs who had filled the Commons benches, including party leader Sir Keir Starmer
The seat was previously held by her sister Jo Cox, who was murdered in the constituency in June 2016.
There was applause from the opposition benches when a grinning Ms Leadbeater gave a thumbs up after she took the oath in front of the Speaker of the House of Commons, Sir Lindsay Hoyle.
She was elected with a majority of just 323 votes last Thursday, beating the Conservative candidate Ryan Stephenson whom many had predicted to win.
She took 13,296 votes compared to Mr Stephenson’s 12,973, while George Galloway came in third with 8,264 ballots.
It followed a campaign marked by accusations of violence and dirty tricks as Labour supporters and Mr Galloway’s Workers Party battled for votes in the constituency’s Asian communities.
Speaking after she was announced as the new MP in the early hours of Friday morning, Ms Leadbeater said: “I’m absolutely delighted that the people of Batley and Spen have rejected division and they’ve voted for hope.”
The result was also celebrated by Labour leader Sir Keir Starmer, calling it “fantastic result for the brilliant and brave” Ms Leadbeater.
“Kim ran a positive campaign of hope in the face of division” he said.
“She will be an outstanding Labour MP for Batley and Spen.”
The by-election was called in Batley and Spen after the constituency’s former Labour MP Tracy Brabin was elected as mayor of West Yorkshire in May.