沙阿或沙赫（波斯语：شاه‎；英语：Shah）是波斯语古代君主头衔的汉译名。“沙阿”在中文文献中又简称为“沙”，如“花剌子模沙”等。波斯语的头衔“沙阿”在历史上为波斯语民族和很多非波斯语民族所使用。尼泊尔国王也叫沙阿。

“沙阿”是古代伊朗高原诸民族的君主头衔，古代波斯人为伊朗高原诸民族的一支，他们缔造的地跨三洲的阿契美尼德王朝波斯帝国的君主，同样采用了“沙阿”作为头衔。其实阿契美尼德王朝波斯帝国的君主头衔的完整形式为xšāyaθiya xšāyaθiyānām，即汉语的“王中之王”，而此头衔与中古波斯语šāhān šāh相一致，šāhān šāh字面上的意思就是“众王之王”。此头衔的现代波斯语形式为shāhanshāh (شاهنشاه)。在希腊语中此头衔被翻译为"βασιλεύς τῶν βασιλέων (basileus tōn basiléōn)"，βασιλεύς正是希腊语“国王”的意思。以希腊人看来，"βασιλεύς τῶν βασιλέων (basileus tōn basiléōn)" 王中之王在国王之上，和“皇帝”是同一等级。此头衔相应的印度语言头衔为rajadhiraja 或kshetra-pati ，后者更接近于 Padishah。正如此波斯语的完整头衔常简化为“沙阿”，希腊人也常将相应的头衔简化为basileus。

Shah (Šâh or Şah) (/ˈʃɑː/; Persian:شاه‎‎, [ʃɒːh], "king") is a title given to the emperors/kings and lords of Iran (historically also known as Persia). It was also adopted by the kings of Shirvan (a historical Iranian region in Transcaucasia) namely the Shirvanshahs, the rulers and offspring of the Ottoman Empire (termed there as Şeh), Mughal emperors of the Indian Subcontinent, the Bengal Sultanate, as well as in Georgia and Afghanistan. In Iran (Persia and Greater Persia) the title was continuously used; rather than King in the European sense, each Persian ruler regarded himself as the Šâhanšâh (King of Kings) or Emperor of the Persian Empire. The word descends from Old Persian Xšâyathiya "king", which (for reasons of historical phonology) must be a borrowing from Median, and is derived from the same root as Avestan xšaΘra-, "power" and "command", corresponding to Sanskrit (Old Indic) kṣatra- (same meaning), from which kṣatriya-, "warrior", is derived. The full, Old Persian title of the Achaemenid rulers of the First Persian Empire was Xšâyathiya Xšâyathiyânâm or Šâhe Šâhân, "King of Kings" or "Emperor". This word is commonly confused with the unrelated and distinct Indian surname Shah, which is derived from the Sanskrit Sadhu/Sahu (meaning gentleman).