Samuel G. Arnold (April 12, 1821 - February 14, 1880) was a United States Senator from Rhode Island. Born in Providence, Rhode Island, he received his early education under private tutors, and graduated from Brown University in 1841, and, in 1845, the law department of Harvard University, gaining admission to the bar that year. He was a lawyer and historian, and was trustee of Brown University from 1848 to 1880.
Arnold was elected lieutenant governor of Rhode Island in 1852 and served as Acting Governor. He was a member of the peace commission held at Washington, D.C. in 1861 in an effort to devise means to prevent the impending war, and was again elected lieutenant governot in 1861 and 1862. During the Civil War he raised a company of light artillery which went to Washington D.C. and was mustered into the Union Army. He was elected as a Republican to the U.S. Senate to fill the vacancy caused by the resignation of James F. Simmons, and served from December 1, 1862, to March 3, 1863. After his time in the Senate he returned to historical research and was president of the Rhode Island Historical Society from 1868 to 1880. Arnold died in Providence in 1880; interment was in Swan Point Cemetery.
Theodore Francis Green, Samuel Arnold's grandnephew, was also a U.S. Senator from Rhode Island.