The California State Telegraph Company was a business originally organized to provide telegraph service between San Francisco and Marysville, California. By the spring of 1861, the company had expanded its service area south to Los Angeles, north to Yreka, and east to Fort Churchill by absorbing the other telegraph companies in California (partly through enforcement of its right to the Morse telegraph patent). In 1861, the company formed the Overland Telegraph Company, which was responsible for constructing part of the telegraph line which resulted in the first transcontinental telegraph network in the United States.
The California State Telegraph Company was absorbed into the Western Union Telegraph Company in 1867, with its lines becoming part of Western Union's Pacific Division.