马库斯·图利乌斯·西塞罗（Marcus Tullius Cicero，前106年1月3日－前43年12月7日，其名在拉丁语中读为[ˈkɪkɛroː]（音译为基凯罗），西塞罗为英文音译，发音： /ˈsɪsᵻroʊ/），是罗马共和国晚期的哲学家、政治家、律师、作家、雄辩家。他出生于骑士阶级的一个富裕家庭，青年投身法律和政治，其后曾担任罗马共和国的执政官；同时，因为其演说和文学作品，他被广泛地认为是古罗马最伟大的演说家和最具影响力的散文作家之一。在罗马共和国晚期的政治危机中，他是共和国所代表的自由主义的忠诚辩护者，马克·安东尼的政敌。他支持古罗马的宪制，因此也被认为是三权分立学说的古代先驱，公元前63年当选为执政官，后被马克·安东尼派人杀害于福尔米亚。

西塞罗因其作品的文学成就，为拉丁语的发展作出了不小的贡献。他在当时是罗马著名的文学人物，其演说风格雄伟、论文机智、散文流畅，设定了古典拉丁语的文学风格。西塞罗也是一位古希腊哲学的研究者。他通过翻译，为罗马人介绍了很多希腊哲学的作品，使得希腊哲学的研究得以在希腊被罗马征服之后得以延续。

西塞罗在古罗马时代的影响在中世纪时代渐渐衰落，但在文艺复兴时被重新振兴。彼特拉克在14世纪重新发现了西塞罗的书信，由此开始了文艺复兴学者对西塞罗的重新研究。因此有学者认为，文艺复兴在本质上是对西塞罗的复兴。西塞罗的影响在启蒙时代达到了顶峰，受其政治哲学影响者包括洛克、休谟、孟德斯鸠等哲学家。亚当斯、汉密尔顿等人也常在其作品中引用西塞罗的作品。

西塞罗深远地影响了欧洲的哲学和政治学说，并且至今仍是罗马历史的研究对象。

Marcus Tullius Cicero (/ˈsɪsəroʊ/ SISS-ə-roh; Latin: [ˈmaːrkʊs ˈt̪ʊlːijʊs ˈkɪkɛroː]; 3 January 106 – 7 December 43 BC) was a Roman statesman, lawyer, scholar and Academic skeptic who played an important role in the politics of the late Roman Republic and in vain tried to uphold republican principles during the crises that led to the establishment of the Roman Empire. His extensive writings include treatises on rhetoric, philosophy and politics, and he is considered one of Rome's greatest orators and prose stylists. He came from a wealthy municipal family of the Roman equestrian order, and served as consul in 63 BC.

His influence on the Latin language was immense: he wrote more than three-quarters of surviving Latin literature from the period of his adult life, and it has been said that subsequent prose was either a reaction against or a return to his style, not only in Latin but in European languages up to the 19th century. Cicero introduced into Latin the arguments of the chief schools of Hellenistic philosophy and created a Latin philosophical vocabulary with neologisms such as evidentia, humanitas, qualitas, quantitas, and essentia, distinguishing himself as a translator and philosopher.

Though he was an accomplished orator and successful lawyer, Cicero believed his political career was his most important achievement. It was during his consulship that the second Catilinarian conspiracy attempted to overthrow the government through an attack on the city by outside forces, and Cicero suppressed the revolt by summarily and controversially executing five conspirators. During the chaotic latter half of the 1st century BC marked by civil wars and the dictatorship of Gaius Julius Caesar, Cicero championed a return to the traditional republican government. Following Julius Caesar's death, Cicero became an enemy of Mark Antony in the ensuing power struggle, attacking him in a series of speeches. He was proscribed as an enemy of the state by the Second Triumvirate and consequently executed by soldiers operating on their behalf in 43 BC after having been intercepted during an attempted flight from the Italian peninsula. His severed hands and head were then, as a final revenge of Mark Antony, displayed on the Rostra.

Petrarch's rediscovery of Cicero's letters is often credited for initiating the 14th-century Renaissance in public affairs, humanism, and classical Roman culture. According to Polish historian Tadeusz Zieliński, "the Renaissance was above all things a revival of Cicero, and only after him and through him of the rest of Classical antiquity." The peak of Cicero's authority and prestige came during the 18th-century Enlightenment, and his impact on leading Enlightenment thinkers and political theorists such as John Locke, David Hume, Montesquieu and Edmund Burke was substantial. His works rank among the most influential in European culture, and today still constitute one of the most important bodies of primary material for the writing and revision of Roman history, especially the last days of the Roman Republic.