Second Empire style, also known as the Napoleon III style, is a highly eclectic style of architecture and decorative arts, which uses elements of many different historical styles, and also made innovative use of modern materials, such as iron frameworks and glass skylights. It flourished in the Second French Empire during the reign of Emperor Napoleon III (1852–1870) and had an important influence on architecture and decoration in the rest of Europe and North America. Major examples of the style include the Opéra Garnier (1862–1871) in Paris by Charles Garnier, the Bibliothèque nationale de France, the Church of Saint Augustine (1860–1871), and the Philadelphia City Hall (1871–1901). The architectural style was closely connected with Haussmann's renovation of Paris carried out during the Second Empire; the new buildings, such as the Opéra, were intended as the focal points of the new boulevards.

The Second French Empire was an Imperial Bonapartist regime of Napoleon III from 14 January 1852 to 27 October 1870, between the Second and the Third French Republics. The period was one of significant achievements in infrastructure and economy, while France reasserted itself as the dominant power in Europe.