馬庫斯·圖利烏斯·西塞羅（Marcus Tullius Cicero，前106年1月3日－前43年12月7日，其名在拉丁語中讀為[ˈkɪkɛroː]（音譯為基凱羅），西塞羅為英文音譯，發音： /ˈsɪsᵻroʊ/），是羅馬共和國晚期的哲學家、政治傢、律師、作傢、雄辯傢。他出生於騎士階級的一個富裕家庭，青年投身法律和政治，其後曾擔任羅馬共和國的執政官；同時，因為其演說和文學作品，他被廣泛地認為是古羅馬最偉大的演說傢和最具影響力的散文作傢之一。在羅馬共和國晚期的政治危機中，他是共和國所代表的自由主義的忠誠辯護者，馬剋·安東尼的政敵。他支持古羅馬的憲製，因此也被認為是三權分立學說的古代先驅，公元前63年當選為執政官，後被馬剋·安東尼派人殺害於福爾米亞。

西塞羅因其作品的文學成就，為拉丁語的發展作出了不小的貢獻。他在當時是羅馬著名的文學人物，其演說風格雄偉、論文機智、散文流暢，設定了古典拉丁語的文學風格。西塞羅也是一位古希臘哲學的研究者。他通過翻譯，為羅馬人介紹了很多希臘哲學的作品，使得希臘哲學的研究得以在希臘被羅馬徵服之後得以延續。

西塞羅在古羅馬時代的影響在中世紀時代漸漸衰落，但在文藝復興時被重新振興。彼特拉剋在14世紀重新發現了西塞羅的書信，由此開始了文藝復興學者對西塞羅的重新研究。因此有學者認為，文藝復興在本質上是對西塞羅的復興。西塞羅的影響在啓蒙時代達到了頂峰，受其政治哲學影響者包括洛剋、休謨、孟德斯鳩等哲學家。亞當斯、漢密爾頓等人也常在其作品中引用西塞羅的作品。

西塞羅深遠地影響了歐洲的哲學和政治學說，並且至今仍是羅馬歷史的研究對象。

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.