Voters in Hamilton, Larkhall and Stonehouse are heading to the polls to elect a new MSP.
A by-election is being held following the death of the SNP's Christina McKelvie.
The polls open at 07:00 and close at 22:00, with a result expected in the early hours of Friday morning.
In what is the first Scottish Parliament by-election since 2019, parties are aiming to build momentum ahead of next year's Holyrood elections.
There are 10 candidates contesting the by-election. They are:
Hamilton, Larkhall and Stonehouse had been held by McKelvie since the constituency was created in 2011.
The SNP MSP served in several government roles, latterly as drugs and alcohol minister.
She died aged 57 in March after being diagnosed with secondary breast cancer.
The new MSP will only be in place for less than a year before the Scottish Parliament elections, scheduled for May 2026.
The by-election will be a first-past-the-post ballot, meaning the candidate with the highest number of votes will become the constituency's new MSP.
Voters do not need ID to cast their ballot.
South Lanarkshire Council has advised those voting by post that if they have not sent their ballot in good time to arrive by 22:00, they can hand them in to a polling station.
The council has published more advice on its website.