The Cathedral of the Protection of Most Holy Theotokos on the Moat (Russian: Собор Покрова пресвятой Богородицы, что на Рву) or Pokrovsky Cathedral (Russian: Покровский собор) are official names for a Russian Orthodox church in Red Square in Moscow. The church is also called the Cathedral of St. Vasily the Blessed (Russian: Собор Василия Блаженного), anglicized as Saint Basil's Cathedral. It was built from 1555–61 on orders from Ivan the Terrible and commemorates the capture of Kazan and Astrakhan.

St. Basil's marks the geometric center of Moscow (not to be confused with the geographic center, the Ivan the Great Bell Tower). It has been the hub of the city's growth since the 14th century and was the city's tallest building until the completion of the Ivan the Great Bell Tower in 1600.

The original building, known as "Trinity Church" and later "Trinity Cathedral", contained eight side churches arranged around the ninth, central church of Intercession; the tenth church was erected in 1588 over the grave of venerated local saint Vasily (Basil). In the 16th and 17th centuries the church, perceived as the earthly symbol of the Heavenly City, as happens to all churches in Byzantine Christianity, was popularly known as the "Jerusalem" and served as an allegory of the Jerusalem Temple in the annual Palm Sunday parade attended by the Patriarch of Moscow and the tsar.