Ortaköy Mosque, also known as the Büyük Mecidiye Mosque, is a historic mosque located in the Ortaköy neighbourhood of Istanbul, Turkey. It is situated on the shores of the Bosphorus Strait, near the Ortaköy Bridge, and is one of the most popular tourist attractions in Istanbul.

Ortaköy Mosque was built in the mid-19th century during the reign of Ottoman Sultan Abdülmecid I. It was designed by the Italian architect Nigoglu, who was inspired by the Baroque style of architecture. 

The mosque is known for its unique architectural style, which combines elements of Ottoman and European Baroque styles. It is built in the shape of a cross, with four large domes and four smaller domes surrounding the central prayer hall. 

The mosque's exterior is adorned with intricate decorations, including marble columns, arches, and reliefs. The mosque's interior is also decorated with beautiful tiles, chandeliers, and other decorative elements.

Ortaköy Mosque is an important cultural and architectural landmark in Istanbul and is a popular tourist attraction. It is also an important religious site for Muslims, as it is one of the city's largest and most important mosques. 

The mosque is open to visitors and is a place of worship for the local community. In addition to its religious and cultural significance, Ortaköy Mosque is a popular spot for tourists and locals to enjoy beautiful views of the Bosphorus Strait and the city of Istanbul.