William Cost Johnson (January 14, 1806 – April 14, 1860) was an American politician.
Johnson was born near Jefferson, Maryland, and studied law.
Johnson served as a member of the Maryland House of Delegates in 1831 and 1832, and was elected as an Anti-Jacksonian to the Twenty-third Congress, serving from March 4, 1833, to March 3, 1835.
After Congress, he continued the practice of his profession until his death in Washington, D.C..
He is interred in the Reformed Church Cemetery in Jefferson.