Balfour Beatty plc (/ˌbælfʊər ˈbiːtiː/) is an international infrastructure group based in the United Kingdom with capabilities in construction services, support services and infrastructure investments. A constituent of the FTSE 250 Index, the company is active across the UK, US and Hong Kong. In terms of turnover, Balfour Beatty was ranked in 2021 as the biggest construction contractor in the United Kingdom.

It was formed on 12 January 1909 by the engineer George Balfour and the accountant Andrew Beatty. Initially working on tramways, the company soon expanded into power and general contracting; the First World War saw it construct several army bases and various other works to support the British war effort. During the 1920s and 1930s, Balfour Beatty reoriented away from bus and tramway operations towards more lucrative heavy civil engineering, particularly the development of Britain's National Grid and various power stations. Early international projects include hydro electric power schemes in the Dolomites, Malaya and India, power stations in Argentina and Uruguay, and the Kut Barrage on the Tigris in Iraq. During the Second World War, the company's construction efforts were dominated by the war effort, including blocking the approaches to Scapa Flow and the building of six Mulberry harbour units.

For a time, Balfour Beatty's activities were dominated by two domestic sectors: power stations and the railways. It also opted to develop its presence as contractor within various power and civil engineering projects. Throughout the 1970s, Balfour Beatty expanded its presence in the road construction sector through schemes such as the M73 motorway and the Glasgow Inner Ring Road. Between 1986 and 1995, Balfour Beatty operated Balfour Beatty Homes; after a collapse of the housing market, Balfour Beatty Homes was renamed Clarke Homes and then sold to Westbury. During the 2000s, the company's business strategy diversified from the construction of infrastructure alone towards the financing, operation, design and management functions. Balfour Beatty also pursued a strategy of growth via acquisition, primarily in the United Kingdom and North America, including Mansell plc, Birse Group, Rok plc, Centex Construction, Parsons Brinckerhoff, and Howard S. Wright.

During the 2010s, several instances of legal action was taken against the company for its alleged use of blacklists. In 2014, Balfour Beatty rebuffed three offers by Carillion, its primary British-based rival at that time, to purchase the company. Throughout the 2010s and 2020s, Balfour Beatty has been heavily involved in several major railway projects in Britain, including High Speed 2, Crossrail, and the modernisation of the Great Western Main Line. In October 2005, Balfour Beatty was found guilty of breaching health and safety laws, and were fined £10 million for its involvement in the October 2000 Hatfield rail crash.

Balfour Beatty was formed on 12 January 1909, with a capital of £50,000. The two founding principals were George Balfour, a qualified mechanical and electrical engineer, and Andrew Beatty, an accountant. The two had met while working for the London branch of the New York engineers JG White & Company. Initially, the company concentrated on tramways, the first contract being to construct the Dunfermline and District Tramways that opened in November 1909 for Balfour Beatty's own subsidiary, the Fife Tramway Light and Power Company.

Balfour Beatty's fortunes were heavily impacted by the outbreak of the First World War. A portion of its staff were drawn away to serve in the British Army. Its expertise in general construction was put to use in the development of numerous army camps, including a large complex at Ripon. It also built an 8km aqueduct at Kinlochleven to supply the works of the British Aluminium Company; this was the company's first endeavour into heavy civil engineering. This contracting work would develop into a lucrative activity for the business.

During its first two decades of operations, the company acquired a portfolio of electric power and tramway companies including in Carlisle, Cheltenham, Leamington & Warwick, Llanelly, Luton, Mansfield, Nottinghamshire & Derby, Falkirk and Wemyss. Some later operated trolley and motor buses. Several bus companies were purchased or formed including Midland General, Percivals (Carlisle), Stratford Blue and Scottish General Omnibus.

The Scottish bus subsidiaries were sold in June 1930 to W Alexander & Sons, and the Scottish tramways in 1935 to Scottish Motor Traction, Cheltenham was sold in July 1939 to Red & White Services with the remaining operations transferred to the Tilling Group.