James Cassius Williamson was an American actor and later Australia's foremost impresario, founding the J. C. Williamson's theatrical and production company.

J. C. Williamson's, formerly Williamson, Garner, & Musgrove and Williamson and Musgrove, was an Australian theatrical management company and theatre owner. With its beginnings in the theatrical productions of J. C. Williamson and his partners in the nineteenth and early twentieth century, the company J. C. Williamson Limited was established in 1910. Colloquially known as The Firm or JCW, the company dominated Australian commercial theatre in the twentieth century and at one time was described as the largest theatrical firm in the world. It closed under financial pressure in 1976.