Ortaköy Mosque (Turkish: Ortaköy Camii) or Büyük Mecidiye Camii (Turkish: Büyük Mecidiye Camii, lit. 'Great Mosque of Sultan Abdulmejid') in Beşiktaş, Istanbul, Turkey, is a mosque situated at the waterside of the Ortaköy pier square, one of the most popular locations on the Bosphorus. It was commissioned by the Ottoman sultan Abdülmecid I and its construction was completed around 1854 or 1856.

This structure is symbolic of the district of Ortaköy as it has a distinctive view of the Bosphorus Strait of Istanbul and the Bosphorus Bridge. The mosque is visible from Bosphorus cruises and ferries.

On the site of the present-day Ortaköy Mosque there was previously a small mosque built in 1720 and ruined during the Patrona Halil Uprising in 1731. The current mosque was commissioned by the Ottoman sultan Abdülmecid I and built or completed around 1854 or 1856 (the exact dates of construction vary between scholarly sources). Its architects were Armenian father and son Garabet Balyan and Nikoğos Balyan, who worked as a team and who also designed the nearby Dolmabahçe Palace and the Dolmabahçe Mosque in 1853–1855.

The single dome of the mosque was originally built using bricks. However, the dome developed cracks throughout time and was ready to collapse, so a new dome was reconstructed using concrete. In 1894, there was an earthquake that damaged the mosque, and it also suffered a minor fire in 1984. Thus, the structure has undergone a number of repair and restoration work in its time. Today, it is in fair shape.