A mosque ( MOSK) or masjid ( MASS-jid, MUSS-; both from Arabic: مَسْجِد, romanized: masjid, pronounced [ˈmasdʒid]; lit. 'place of ritual prostration') is a place of prayer for Muslims. Mosques are usually covered buildings, but can be any place where prayers (sujud) are performed, including outdoor courtyards.

The first mosques were simple places of prayer for Muslims, and may have been open spaces rather than buildings. In the first stage of Islamic architecture, 650-750 CE, early mosques comprised open and closed covered spaces enclosed by walls, often with minarets from which calls to prayer were issued. Mosque buildings typically contain an ornamental niche (mihrab) set into the wall that indicates the direction of Mecca (qiblah) and ablution facilities. The pulpit (minbar), from which the Friday (jumu'ah) sermon (khutba) is delivered, was in earlier times characteristic of the central city mosque, but has since become common in smaller mosques. Mosques typically have segregated spaces for men and women. This basic pattern of organization has assumed different forms depending on the region, period and denomination.

Mosques commonly serve as locations for prayer, Ramadan vigils, funeral services, marriage and business agreements, alms collection and distribution, as well as homeless shelters. Historically, mosques have served as a community center, a court of law, and a religious school. In modern times, they have also preserved their role as places of religious instruction and debate. Special importance is accorded to the Great Mosque of Mecca (centre of the hajj), the Prophet's Mosque in Medina (burial place of Muhammad) and Al-Aqsa Mosque in Jerusalem (believed to be the site of Muhammad's ascent to heaven).

With the spread of Islam, mosques multiplied across the Islamic world. Sometimes churches and temples were converted into mosques, which influenced Islamic architectural styles. While most pre-modern mosques were funded by charitable endowments, increasing government regulation of large mosques has been countered by a rise of privately funded mosques, many of which serve as bases for different Islamic revivalist currents and social activism. Mosques have played a number of political roles. The rates of mosque attendance vary widely depending on the region.