Arecibo (Spanish pronunciation: [aesio]) is a city and municipality on the northern coast of Puerto Rico, on the shores of the Atlantic Ocean, located north of Utuado and Ciales; east of Hatillo; and west of Barceloneta and Florida. It is about 50 miles (80 km) west of San Juan, the capital city. Arecibo is the largest municipality in Puerto Rico by area, and is part of the San Juan, Caguas and Guaynabo Metropolitan Statistical Area. It is spread over 18 barrios and Arecibo Pueblo (the downtown area and the administrative center of the city). Its population in 2020 was 87,754.

The Arecibo Observatory, which housed the Arecibo telescope, the world's largest radio telescope until July 2016, is located in the municipality. The Arecibo telescope collapsed on December 1, 2020. Arecibo is the seat of the Roman Catholic Diocese of Arecibo.

The Arecibo Observatory, also known as the National Astronomy and Ionosphere Center (NAIC) and formerly known as the Arecibo Ionosphere Observatory, is an observatory in Barrio Esperanza, Arecibo, Puerto Rico owned by the US National Science Foundation (NSF).

The observatory's main instrument was the Arecibo Telescope, a 305 m (1,000 ft) spherical reflector dish built into a natural sinkhole, with a cable-mount steerable receiver and several radar transmitters for emitting signals mounted 150 m (492 ft) above the dish. Completed in 1963, it was the world's largest single-aperture telescope for 53 years, surpassed in July 2016 by the Five-hundred-meter Aperture Spherical Telescope (FAST) in China. Following two breaks in cables supporting the receiver platform in mid-2020, the NSF decommissioned the telescope. A partial collapse of the telescope occurred on December 1, 2020, before controlled demolition could be conducted. The remains of the telescope are being removed as NASA evaluates plans for a replacement instrument.

The observatory also includes a smaller radio telescope, a LIDAR facility, and a visitor center, which remain operational after the telescope's collapse.