The remains that have been found indicate that the area where Terrassa stands has been inhabited since prehistory. In 2005, during the construction of a tunnel for one of the city's railway lines, a prehistoric site was found in the Park of Vallparadís, with stone tools and fossils of hunted animals dating back 800,000 to 1,000,000 years, making this is one of the oldest prehistoric sites in Europe.

Terrassa originated as the Roman town of Egara (Municipium Flavium Egara), which was founded during the time of the emperor Vespasian (69-79 CE) alongside the torrent of Vallparadís (nowadays an urban park) close to the Iberian town of Egosa, on the site of which some ceramics and coins have been found.

The episcopal see of Ègara already existed by about 450 CE, and a Provincial Council was held there in 615. The cathedral continued to exist until 718, when it was taken over during the Muslim conquest. Today the ancient Visigothic-Romanesque churches of Sant Pere de Terrassa stand on the site. Other important remains from the Middle Ages are the castle of Vallparadís (from 1344 to 1413 a Carthusian monastery and today a municipal museum) and the tower of the castle-palace of the count-king).

In the 19th century the city played an important role in the industrial revolution, specializing in woollen fabrics, and today there is a major Modernista legacy as a result of the city's importance at that time. Particularly notable Modernista buildings include the Masia Freixa (1907), the Vapor Aymerich, Amat i Jover textile mill (1907) (now the Museum of Science and Industry of Catalonia), the Principal theater (1920), the city hall (1902), the Alegre de Sagrera house/museum (1911), the Industrial School (1904), the Gran Casino (1920), the Parc de Desinfecció (1920), and the Independència market (1908). Terrassa is a partner city of the Art Nouveau network, a European network of co-operation created in 1999 for the study, preservation and development of Art Nouveau.