Castelldefels is a municipality located in the Baix Llobregat comarca within the province of Barcelona, Catalonia, Spain. It serves as a suburban town in the Metropolitan Area of Barcelona, situated approximately 20 kilometers southwest of the city center. The town lies just south of the massís del Garraf and represents the last coastal settlement before entering the comarca of Garraf. Its geographic position places it along the Mediterranean Sea coast between the neighboring towns of Sitges and Gavà.

The town's history is marked by its strategic military importance during the early middle ages, when it stood on the frontier between the County of Barcelona and lands controlled by the Moors. A number of castles were built to protect this border, with Castelldefels Castle being one of the most significant. The oldest parts of this castle date from the 10th century, reflecting its long-standing role in regional defense and governance throughout the medieval period.

Castelldefels is defined by its extensive coastline, featuring a famous beach that stretches for more than five kilometers. The town is known as an affluent area and has been a residence for numerous famous sportsmen, including Lionel Messi, Luis Suárez, Philippe Coutinho, and formerly Ronaldinho. Key landmarks include the Castelldefels Castle, which now hosts a museum dedicated to piracy, and the Olympic canal, known as the Canal Olímpic de Catalunya, which was constructed for the 1992 Summer Olympic Games.

The municipality attracts visitors with its gastronomy and popular beach culture, drawing many people from Barcelona and the countryside during the summer. The town inspired the creation of the character El Neng de Castefa, who appeared on the late-night show BFN hosted by Andreu Buenafuente. Visitors can explore local dining options such as Restaurante Olave Plaza, known for fresh seafood, and Restaurante Cel Blau, famous for its paellas, alongside the historic castle museum.

The population of Castelldefels is recorded at 70,057 as of 2025. The town offers robust transport links, including two railway stations on the Renfe R2 line and bus routes connecting to Barcelona and El Prat Airport, which is about 15 kilometers away. Educational facilities include the Castelldefels School of Technology and campuses of the British School of Barcelona, supporting a diverse community in this coastal suburb.