Scotland TranServ, a joint venture of Balfour Beatty and Mouchel, will start on 1 April 2013. The contract has a total potential value in excess of £125m over the initial five-year term.
There are options to extend the deal for up to five further years. It will be worth approximately £25m a year.
The client is the Scottish government, through Transport Scotland.
The South West region covers an area stretching from Stranraer in Dumfries & Galloway to Alexandria in Strathclyde. It includes 681km of roads and 1,912 structures.
Balfour Beatty chief executive Ian Tyler said: “We are delighted to have been selected by the Scottish ministers to deliver this contract to maintain roads on routes vital to the country’s economy. We look forward to continuing to work closely with Transport Scotland, building on our successes and bringing innovative and efficient approaches to improving trunk road maintenance.”