Gustav Thomassen (16 February 1862 – 6 May 1929) was a Norwegian stage actor and theatre director.

He was the son of Salmaker Osmund Thomassen (1816-1892) and Caroline Olava Thomine Eckhoff (1828-1871).

He worked at Den Nationale Scene until 1905, and served as theatre director from 1900 to 1905.

He was noted for his title role in the four act comedy Jan Herwitz.

His main effort at the Nationaltheater was the staging of comedies by Ludvig Holberg, especially Jeppe på bjerget (1722), Den Stundesløse (1723) Erasmus Montanus (1722) and Barselstuen (1723).