Midlothian South, Tweeddale and Lauderdale is a county constituency of the Scottish Parliament covering parts of the council areas of Midlothian and Scottish Borders. Under the additional-member electoral system used for elections to the Scottish Parliament, it elects one Member of the Scottish Parliament (MSP) by the first past the post method of election. It is also one of ten constituencies in the South Scotland electoral region, which elects seven additional members, in addition to the ten constituency MSPs, to produce a form of proportional representation for the region as a whole.
Created for the 2011 election, the constituency covers parts of the former constituencies of Midlothian and Tweeddale, Ettrick and Lauderdale. The remainder of Midlothian council area lies within the constituency of Midlothian North, whilst the remainder of Scottish Borders forms the Ettrick, Roxburgh and Berwickshire constituency.
Following the second periodic review of Scottish Parliament boundaries in 2025, the other nine constituencies of the South Scotland region are: Ayr; Carrick, Cumnock and Doon Valley; Clydesdale; Dumfriesshire; East Kilbride; Ettrick, Roxburgh and Berwickshire; Galloway and West Dumfries; Hamilton, Larkhall and Stonehouse; and Kilmarnock and Irvine Valley. The region covers the whole of the council areas of Dumfries and Galloway, Scottish Borders, and South Ayrshire council areas; and parts of the council areas of East Ayrshire, Midlothian, and South Lanarkshire. By population it is now the largest of Scotland's eight electoral regions.
Prior to the 2025 review, there were nine constituencies in the South Scotland region. Besides Ettrick, Roxburgh and Berwickshire, the other eight constituencies were: Ayr; Carrick, Cumnock and Doon Valley; Clydesdale; Dumfriesshire; East Lothian; Ettrick, Roxburgh and Berwickshire; Galloway and West Dumfries; and Kilmarnock and Irvine Valley. The region covered the Dumfries and Galloway, East Ayrshire, Scottish Borders and South Ayrshire council areas in full and parts of the East Lothian, Midlothian and South Lanarkshire council areas.
Midlothian North, which covers the rest of Midlothian, is part of the Edinburgh and Lothians East region.
Midlothian South, Tweeddale and Lauderdale was created by the First Periodic Review of Scottish Parliament Boundaries ahead of the 2011 election from parts of the former constituencies of Midlothian and Tweeddale, Ettrick and Lauderdale. Following the Second Periodic Review in 2025, the boundary with Midlothian North was ammended to move Newtongrange into Midlothian North. The electoral wards of Scottish Borders Council and Midlothian Council used in the current creation of Midlothian South, Tweeddale and Lauderdale are: