Casa Batlló is a building designed by the architect Antoni Gaudí, the leading representative of Catalan Modernisme. It is a complete remodelling of a previously existing building on the site, the work of Emilio Sala Cortés.

It is located at number 43 Passeig de Gràcia in Barcelona, the wide avenue that crosses the Eixample district, in the so-called "Manzana de la discordia" (Block of Discord), because in addition to this building it houses other works by modernist architects: Casa Amatller, which adjoins Gaudí's, the work of Josep Puig i Cadafalch; Casa Lleó Morera, by Lluís Domènech i Montaner; Casa Mulleras, by Enric Sagnier; and Casa Josefina Bonet, by Marceliano Coquillat. Construction took place between 1904 and 1906.

The building was constructed in 1875 by Emilio Sala Cortés (1841-1920), an architect who designed several buildings in Barcelona and the province. Sala was also a professor at the Barcelona School of Architecture and was one of Gaudí's teachers, whom he occasionally employed as a draughtsman.

In 1903 the building was acquired by the industrialist Josep Batlló, a wealthy businessman who owned several textile factories in Barcelona. Batlló bought the building with the initial intention of demolishing it and building a new one, although he later settled for refurbishing it, and while he kept the main floor for himself, he rented out the rest, as was customary in the bourgeois houses of the time.

For the remodelling, Batlló commissioned the project to Gaudí, by then an architect of great renown, who at that time was working on several projects at the same time. The industrialist was so impressed by the architect's work that he recommended him to his friend Pedro Milà, for whom Gaudí built the Casa Milà.

The building has eight floors: below street level there is a basement, used for coal bunkers and storage rooms; the ground floor was initially used as a garage and then as a warehouse, and next to the entrances to the flats there was a shop.

The crowning feature of the building is the roof terrace, which is accessed from the attic by a spiral staircase. Gaudí conceived this space in a functional way, to locate the smoke and ventilation outlets, but at the same time aesthetically pleasing, as the elements that make it up are handcrafted and almost sculptural in their plasticity.