Raphael Semmes (/sɪmz/ SIMZ; September 27, 1809 – August 30, 1877) was an officer in the Confederate Navy during the American Civil War. He had served as an officer in the United States Navy from 1826 to 1860.

During the American Civil War, Semmes was captain of the cruiser CSS Alabama, the most successful commerce raider in maritime history, taking 65 prizes. Late in the war, he was promoted to rear admiral. He also acted as a brigadier general in the Confederate States Army from April 5 to April 26, 1865, although this appointment was never submitted to or officially confirmed by the Confederate Senate.

Semmes was born in Charles County, Maryland, on Tayloe's Neck. He was a cousin of future Confederate general Paul Jones Semmes and of future Union Navy Captain Alexander Alderman Semmes.^{[citation needed]}

He graduated from Charlotte Hall Military Academy and entered the U.S. Navy as a midshipman in 1826. Semmes first served on the Lexington, cruising the Caribbean and the Mediterranean until September 1826, when he was placed on leave for ill health. After a short convalescence, he served on the USS Erie for part of 1829 and on the USS Brandywine (formerly Susquehanna) for the rest of 1829 and the first nine months of the following year. On September 29, 1830, he was posted to the USS Porpoise of the West Indies squadron, which was attempting to suppress piracy in the Caribbean. Semmes then studied law and was admitted to the bar. He was promoted to lieutenant in February 1837.

During the Mexican–American War, he commanded the USS Somers in the Gulf of Mexico. In December 1846, a squall hit the ship while under full sail in pursuit of a vessel off Veracruz. Somers capsized and was lost along with 37 sailors. Semmes then served as first lieutenant on the USS Raritan, accompanied the landing force at Veracruz, and was dispatched inland to catch up with Army forces proceeding to Mexico City.

Following the war, Semmes went on extended leave at Mobile, Alabama, where he practiced law and wrote Service Afloat and Ashore During the Mexican War. He became extremely popular, and the nearby town of Semmes, Alabama, was named after him. He also maintained a home in Josephine, Alabama, on Perdido Bay. He was promoted to commander in 1855 and was assigned to lighthouse duties until 1860. After Alabama seceded from the Union, Semmes was offered a Confederate naval appointment by the provisional government; he resigned from the U.S. Navy the next day, February 15, 1861.

After appointment to the Confederate Navy as a commander and a futile assignment to purchase arms in the North, Semmes was sent to New Orleans to convert the steamer Habana into the cruiser/commerce raider CSS Sumter. In June 1861, Semmes, in Sumter, outran USS Brooklyn, breaching the Union blockade of New Orleans, and then launched a brilliant career as one of the greatest commerce raider captains in naval history.

Semmes' command of Sumter lasted only six months, but during that time he ranged wide, raiding US commercial shipping in both the Caribbean Sea and Atlantic Ocean; his actions accounted for the loss of 18 merchant vessels, while always eluding pursuit by Union warships. By January 1862, Sumter required a major overhaul. Semmes' crew surveyed the vessel while in neutral Gibraltar and determined that the repairs to her boilers were too extensive to be completed there. Semmes paid off the crew and laid up the vessel. US Navy vessels maintained a vigil outside the harbor until she was disarmed and sold at auction in December 1862, eventually being renamed and converted to a blockade runner.