沙皇（俄语：царь）是莫斯科大公国及俄罗斯帝国最高统治者1547年到1721年的正式称呼。

沙皇为音译意译合词。“沙”实际上是东罗马帝国（拜占庭）皇帝的名号“凯撒”Caesar。“凯撒”音译为沙，加一皇字，以表示皇帝之意。俄罗斯人对蒙古可汗也敬称“凯撒”。土耳其帝国灭拜占庭后，莫斯科大公自认为是拜占庭皇帝法统的继承人，使用凯撒的称号，汉语译作沙皇。在清代早期的汉文、蒙古文书中，沙皇被称作“察罕汗”。“察罕”为蒙古语白色之意，察罕汗意为“白人的可汗”。

俄罗斯帝国自彼得大帝（1721年）开始，就放弃“凯撒”，改用「皇帝」（俄语：Император）作为君主的主要称号，但在俄国民间与各国仍一直习惯把俄罗斯皇帝都俗称为沙皇。

Tsar /zɑːr/ (Old Church Slavonic: ц︢рь (usually written thus with a title) or цар, цaрь; Tsar in Latin alphabet languages) is a title used to designate certain European Slavic monarchs or supreme rulers. As a system of government in the Tsardom of Russia and Russian Empire, it is known as Tsarist autocracy, or Tsarism. The term is derived from the Latin word Caesar, which was intended to mean "Emperor" in the European medieval sense of the term—a ruler with the same rank as a Roman emperor, with-holding it by the approval of another emperor or a supreme ecclesiastical official (the Pope or the Ecumenical Patriarch)—but was usually considered by western Europeans to be equivalent to king, or to be somewhat in between a royal and imperial rank.