Spencer Darwin Pettis (1802 – August 28, 1831), U.S. Representative from Missouri and the fourth Missouri Secretary of State. He is best known, however, for being a participant in a fatal duel with Major Thomas Biddle. Pettis County, Missouri, is named in his honor.

What transpired on August 26, 1831, had its roots many months earlier. Missouri's Jacksonian Democrats, led by Senator Thomas Hart Benton engaged in a number of debates during the 1830 Congressional election season that saw many fiery speeches on issues of banking, currency stability, and western land use. During one of those speeches Congressman Pettis harshly criticizedNicholas Biddle, President of the Second Bank of the United States. U.S. Army Major Thomas Biddle, a resident of the St. Louis area and brother of Nicholas Biddle, took offense at the remarks.

A war of words soon ensued in the St. Louis press in the form of letters to the editor. In one such letter Biddle called Pettis "a dish of skimmed milk", to which Pettis responded by questioning Biddle's manhood. Pettis was reelected to U.S. Congress in November, 1830 but the feud between the two refused to die. Things escalated dramatically on July 9, 1831, when Thomas Biddle heard that an ill Pettis was resting in a St. Louis hotel. Biddle attacked Pettis in his room, beating him severely with a cowhide whip until other hotel guests could intervene. 

Fearing that he might be attacked again during his time of recovery, Congressman Pettis turned to the judicial system for protection and had Major Biddle arrested on a peace warrant. At the court proceedings Pettis attempted to draw a pistol, with intent of shooting Biddle, until restrained by friends. At this, Biddle stated that he would promptly accept any challenge that the Congressman cared to issue. After sufficient time to recover from the beating, on August 21, 1831, Congressman Pettis challenged Biddle to a duel, which was promptly accepted. As the challenged party, Biddle was allowed to choose the weapons and distance. Being nearsighted, Major Biddle chose pistols at the unusually close distance of five feet. This meant that each man would take at most one or two steps before turning to fire, with their pistols perhaps even overlapping dependent on arm length. In short, it was suicidal and seen as a ploy by some observers and later historians to make Pettis back down thus lose the affair of honor without bloodshed.

At five p.m. on August 27, 1831, Biddle and Pettis, along with their seconds, Major Benjamin O'Fallon and Captain Martin Thomas respectively, met on Bloody Island, a small sandbar located in theMississippi River between St. Louis and the Illinois shore. Dueling was illegal in both states, but authorities tended to turn a blind eye to this neutral ground. As large crowds watched from the St. Louis riverfront, Biddle and Pettis obeyed the commands to step, turn, and fire. When the smoke cleared both men had fallen with mortal wounds. Before being carried off the island both men were overheard to forgive each other for the altercation. Congressman Spencer Pettis died the next day, August 28, while Major Biddle lingered on until August 29. Both men were buried with full honors, eulogized for choosing death before dishonor. The funerals for both men were said to be the largest ever held in St. Louis in the 19th century.

Spencer Pettis had never married and had no children.