Scottish Labour has taken Hamilton, Larkhall and Stonehouse from the SNP in a closely fought by-election.
The party’s Davy Russell won with 8,559 votes, with a majority of 602 over the SNP.
The vote followed the death of SNP MSP Christina McKelvie, who had represented the constituency since 2011.
Scottish Labour leader Anas Sarwar said his party had “proven the pollsters wrong”.
Speaking on the morning after his victory, Russell said he would put the “message of the people” across at Holyrood.
He said: “For far too long the people of Hamilton, Larkhall and Stonehouse, in fact across Scotland, they’ve been listening to the politicians – but the politicians haven’t delivered for them.”
Prime Minister Sir Keir Starmer congratulated Russell, saying: “People in Scotland have once again voted for change.
“Next year there is a chance to turbo charge delivery by putting Labour in power on both sides of the border.”
Reform UK finished third in the by-election on 7,088 votes, with the Tories in fourth on 1,621.
The turnout was 44.2%, with more than 27,000 votes cast. It is a drop on the 2021 election when turnout was 60%.
The result means Scottish Labour has overturned a majority of 4,582 won by Christina McKelvie in 2021.
She died aged 57 in March after being diagnosed with secondary breast cancer.
The by-election result follows a difficult period for Scottish Labour since it swept the SNP aside in last summer’s general election.
The party had seen its support drop in the polls, with some suggesting Reform could beat Labour to second place in next year’s Holyrood elections.
Ahead of the vote, First Minister John Swinney had claimed it was a “two-horse race” between the SNP and Reform.
But his party’s candidate, Katy Loudon, endured her third election defeat in the area – having lost out to Labour’s Michael Shanks at the 2024 general election and 2023 Rutherglen and Hamilton West by-election.
After the result, Swinney said his party had “made progress since the election last year but not enough”. He posted on X: “We still have work to do and we will do it.”
Scottish Constitution Secretary Angus Robertson said the SNP would need to “look very closely” at result and “learn the lessons” for the Holyrood elections.
Davy Russell, who was raised and still lives in the village of Quarter in the constituency, has worked for local government and previously served as deputy lord lieutenant, representing the monarch at local events.
His victory comes after criticism during the campaign for a lack of media appearances – with political opponents dismissing him as “the invisible man”.
Russell said constituents had been “let down” by the SNP.
He added that the result sent a message to Reform leader Nigel Farage that his party’s “poison” was not welcome in Scotland.