James Cassius Williamson (26 August 1845 – 6 July 1913) was an American actor and later Australia's foremost impresario, founding the J. C. Williamson's theatrical and production company.

Born in Pennsylvania, Williamson moved with his family to Milwaukee, Wisconsin. His father died when he was eleven years old. He acted in amateur theatricals and joined a local theatre company as a call-boy at the age of 15, soon taking roles and eventually moving to New York where he played for several years at Wallack's Theatre and then other New York theatres. In 1871, he became the leading comedian at the California Theatre in San Francisco and the next year married comedian and actress Maggie Moore.

The two found success touring in Australia, and then playing in London, the U.S. and elsewhere in a melodrama called Struck Oil. In 1879, Williamson obtained the right to present H.M.S. Pinafore and then other Gilbert and Sullivan operas in Australia. He soon formed his Royal Comic Opera Company. In 1882, Williamson became the lessee of the Sydney Theatre Royal, and that year he entered into a partnership with Arthur Garner and George Musgrove, expanding to own more theatres such as the Adelaide Theatre Royal in 1886, and bringing famous performers to Australia, such as Sarah Bernhardt, Catherine Bartho and H. B. Irving, and becoming known for spectacular, large-scale productions.

After 1907, Williamson moved his family to Europe and, his old partners having left, he hired capable managers and changed the theatre company's name to J. C. Williamson Ltd. Williamson died in 1913, but he left a strong theatrical empire that became the largest theatrical firm in the world, with extensive film and property holdings. The company continued to produce seasons of Gilbert and Sullivan operas, operetta, musical comedy, straight plays, pantomimes and occasional musical revues, and later grand opera, ballet seasons, and concert tours by visiting celebrity singers and musicians, at the many theatres that it owned or leased throughout Australia and New Zealand. It also toured and presented shows in London and elsewhere. In 1976, the company closed and leased out its name.