Freeman Wills Crofts FRSA (1 June 1879 – 11 April 1957) was an Irish engineer and mystery author, remembered best for his Inspector French novels, starting with Inspector French's Greatest Case (1924).

A railway engineer by training, Crofts introduced railway themes into many of his stories, which were notable for their intricate planning. Although Raymond Chandler, Agatha Christie, and authors of the so-called golden age of detective fiction are more famous, he was esteemed by those authors, and many of his books are still in print.

Crofts was born at 26 Waterloo Road, Dublin, Ireland. His father, also named Freeman Wills Crofts, was a surgeon-lieutenant in the Army Medical Service but he died of fever in Honduras before the young Freeman Wills Crofts was born. In 1883, Crofts' mother, née Celia Frances Wise, married the Venerable Jonathan Harding, Vicar of Gilford, County Down, later Archdeacon of Dromore, and Crofts was raised in the vicarage at Gilford. He attended Methodist College and Campbell College in Belfast. In 1912 he married Mary Bellas Canning, daughter of the manager of the Coleraine branch of the Provincial Bank.

In 1896, at the age of seventeen, Crofts was apprenticed to his maternal uncle, Berkeley Deane Wise, who was chief engineer of the Belfast and Northern Counties Railway. In 1899 Crofts was appointed Junior Assistant on the construction of the Strabane–Derry extension of the Donegal Railway Company. In 1900, he became District Engineer at Coleraine for the Northern Counties Committee of the London, Midland and Scottish Railway at a salary of £100pa, living at 11 Lodge Road in the town. In 1922, Crofts was promoted to Chief Assistant Engineer of the railway, based in Belfast. He lived at 'Grianon' in Jordanstown, a quiet village some six miles north of Belfast, to travel by train each day to the railway's offices at York Road. One of the projects he worked on was the design of the Bleach Green Viaduct in Whiteabbey, close to his Jordanstown home. This was a significant 10-arch reinforced concrete viaduct approved in 1927 and completed in 1934. It carried a new loop line which eliminated the need for trains between Belfast and the north west to reverse at Greenisland. Croft continued his engineering career until 1929. In his last task as an engineer, he was commissioned by the Government of Northern Ireland to chair an inquiry into the Bann and Lough Neagh Drainage Scheme.

In 1919, during an absence from work due to a long illness, Crofts wrote his first novel, The Cask (1920), which established him as a new master of detective fiction. Crofts continued to write steadily, producing a book almost every year for thirty years, in addition to a number of short stories and plays.

He is remembered best for his fictional detective, Inspector Joseph French, who was introduced in his fifth book, Inspector French's Greatest Case (1924). Inspector French always solved each of the mysteries presented him in a workmanlike, precise manner – this method set him apart from most other fictional sleuths.

In 1929, Crofts abandoned his railway engineering career and became a full-time writer. He settled in the village of Blackheath, near Guildford, in Surrey, and a number of his books are set in the Guildford area, including The Hog's Back Mystery (1933) and Crime at Guildford (1935). Many of his stories have a railway theme, and his particular interest in the apparently unfalsifiable alibi often emphasized the intricacies of railway timetables. Near the end of his life, he and his wife relocated to Worthing, Sussex in 1953, where they lived until his death in 1957, the year in which his last book was published.

Crofts also wrote one religious book, The Four Gospels in One Story, several short stories, and short plays for the BBC.