William Marmaduke Kavanaugh, (1866-1915), was a Democratic United States Senator from the State of Arkansas.

William Marmaduke Kavanaugh was born near Eutaw, Green County, Alabama on 3 March 1866. Kavanaugh attended public schools in Kentucky and graduated from the Kentucky Military Institute at Farmdale, Kentucky in 1885.

Kavanaugh moved to Little Rock, Arkansas where he worked as a newspaper reporter and later as editor and manager of the Arkansas Gazette.

Kavanaugh then entered politics and was elected sheriff and tax collector of Pulaski County, Arkansas where he served from 1896 to 1900. He then served as a county and probate judge from 1900 to 1904. He then turned his attention to business and began a varied career banking, street railway, and gas supply. From 1912 to 1915 he served as a member of the Democratic National Committee.

Kavanaugh was appointed to the United States Senate to fill the vacancy caused by the death of Jeff Davis. He served in the Senate from 29 January until 3 March 1913. He later served as director of the Lakes to Gulf Deep Waterways Association .

William Marmaduke Kavanaugh died in Little Rock, Arkansas on 21 February 1915. and is buried in Oakland Cemetery in Little Rock.