Nahdlatul Ulama (Indonesian pronunciation: [nahˈdatʊl ʊˈlama], lit. 'Revival of the Ulama', NU) is an Islamic organization in Indonesia. Its membership estimates range from 40 million (2013) to over 95 million (2021), making it the largest Islamic organization in the world. NU is also a charitable body funding schools and hospitals as well as organizing communities to help alleviate poverty.

The NU was founded in 1926 by the ulema and merchants to defend both traditionalist Islamic practices (in accordance with Shafi'i school) and its members' economic interests. NU's religious views are considered "traditionalist" in that they tolerate local culture as long as it doesn't contradict Islamic teachings. By contrast the second largest Islamic organization in Indonesia, the Muhammadiyah, is considered "reformist" as it takes a more literal interpretation of the Qur'an and Sunnah.

Some leaders of Nahdlatul Ulama are ardent advocates of Islam Nusantara, a distinctive variety of Islam that has undergone interaction, contextualization, indigenization, interpretation, and vernacularization according to socio-cultural conditions in Indonesia. Islam Nusantara promotes moderation, anti-fundamentalism, pluralism, and, to a degree, syncretism. Many NU elders, leaders, and religious scholars, however, have rejected Islam Nusantara in favor of a more conservative approach.