Swan Marsh is a rural locality 20 km south-west of Colac in western Victoria. The nearest settlement is Pirron Yallock, and for some years Swan Marsh was known as Pirron Yallock West. It is situated at a place where fertile flat plains merge westwards with stony rises.
Until the early 1900s the area was occupied by large pastoral properties, but closer-settlement farms brought an influx of population. A school was opened in the local hall in 1903. In 1916 the Victorian municipal directory recorded Swan Marsh as having a branch factory of the Colac Dairying Company (1902) and more than one store. A local cheese factory was opened in 1933.
During the years after World War II branch factories were closed as road transport carried more milk over greater distances. Swan Marsh had a hall and the school for the district, which in 1998 had 30 pupils, in 2010 had 9 and closed in 2011.
The Swan Marsh and District Township plan was developed in 2001 with an emphasis on maintaining a picturesque township, both for residents and visitors.
Swan Marsh's census populations have been: