In 1854 Old Courthouse Square was donated to the County of Sonoma by J. Carrillo, B. Hoen, J.W. Hartman and T.G. Hahman. That same year after a vigorous selection campaign, the county seat was moved from Sonoma to Santa Rosa. For the next 30 years the seat of the county government was located adjacent to this Square. In 1884 the first Courthouse on this site was dedicated by General Vallejo. The earthquake of 1906 destroyed that building and much of Santa Rosa. The cornerstone for the last courthouse on this square was laid in 1908. Sonoma County gradually outgrew this structure and in 1966 it was demolished. Its granite steps have been used in the construction of this Square.
This area served as the center for the Administration of Justice in Sonoma County for 82 years. To commemorate its long use as the legal center for the County, it has been officially named 'Old Courthouse Square'.
The above dedication is taken from the plaque donated by the Lawyers Wives of Sonoma County in 1967

From 1967-2016, Old Courthouse Square was divided by a road connecting Santa Rosa Avenue and Mendocino Avenue.
 
In 2017, Old Courthouse Square was reunified and reconstructed to create a civic gathering place in the heart of downtown Santa Rosa. The granite steps were once again saved and used to form the frame around the lawn which is representative of the footprint of the original Courthouse on the Square.