The Municipality of Trebnje (Slovene: Občina Trebnje) is a municipality in Slovenia in the traditional region of Lower Carniola. The seat of the municipality is the town of Trebnje. The municipality was established on 3 October 1994. It was increased in size on 6 August 1998, when the settlements of Kostanjevica and Ravne nad Šentrupertom were transferred to the municipality from the neighboring Municipality of Litija. It was then reduced in size on 13 June 2006 with the formation of the new municipalities of Mokronog–Trebelno and Šentrupert from the municipality's territory.