Measuring three hectares (30,000m²), this is by far the largest Portuguese Square. Designed by Cottinelli Telmo and Vasco Lacerda Marques in 1940 for the Portuguese World Exhibition, it has remained unaltered ever since. In 1956 it lost the two pavilions on its eastern and western ends, and was only closed down on the western end again in 1992 with the construction of the Belém Cultural Centre, where the Pavilion of the Portuguese in the World once stood.

The Imperial Square still preserves the monumental and luminous fountain from its original construction, as well as the ponds on the south end, and a group of sculptures representing mythical sea horses.