The London Midland and Scottish Railway (LMS) was a British railway company. It was formed on 1 January 1923 under the Railways Act of 1921, which required the grouping of over 120 separate railway companies into just four.

The resulting company was an unwieldy construction, with numerous interests other than railway operations. Besides being the world's largest transport organisation, it was also the largest commercial undertaking in the British Empire and the United Kingdom's second largest employer, after the Post Office. The LMS also claimed to be the largest joint stock organisation in the world.

In 1938, the LMS operated 6,870 miles (11,056 km) of railway (excluding its lines in Northern Ireland), but its profitability was generally disappointing, with a rate of return of only 2.7%. Along with the other members of the "Big Four" British railway companies (GWR, LNER and SR), the LMS was nationalised on 1 January 1948, becoming part of the state-owned British Railways.

The LMS was the largest of the Big Four railway companies and the only one to operate in all parts of the British Isles: England, Ireland, Scotland and Wales.