4337 Arecibo (prov. designation: 1985 GB) is a binary asteroid in the outer regions of the asteroid belt, approximately 24 kilometers (15 miles) in diameter. It was discovered on 14 April 1985, by American astronomer Edward Bowell at the Anderson Mesa Station near Flagstaff, Arizona, in the United States. It was named after the Arecibo Observatory in Puerto Rico. It has a relatively large minor-planet moon that was discovered in stellar occultation observations by David Gault and Peter Nosworthy in May 2021, distinguishing it as the first asteroid moon discovered and confirmed solely using the occultation method.

This asteroid was discovered by American astronomer Edward Bowell while measuring a pair of images taken with the Lowell Observatory's 0.33-meter astrograph on 14 April 1985, located at the Anderson Mesa Station near Flagstaff, Arizona, in the United States. The discovery observations were published by the Minor Planet Center on 4 May 1985. In September 1985, astronomers Kiichirō Furukawa and Lutz D. Schmadel both identified the asteroid in prediscovery observations from March and April 1979, while Furukawa independently found earlier prediscovery observations from April 1933. With these prediscovery observations linked, the asteroid's orbit was significantly improved and eventually received its permanent minor planet number of 4337 from the Minor Planet Center on 11 January 1990.

The asteroid was named after the Arecibo Observatory in Puerto Rico, home to the world's largest filled-aperature radio telescope in the 20th century. The name was proposed by radar astronomer Steven J. Ostro, in recognition of the observatory's indispensable contributions to the characterization of Solar System bodies including asteroids. The official naming citation was published by the Minor Planet Center on 8 June 1990.

On 19 May 2021, two amateur astronomers, David Gault and Peter Nosworthy, observed Arecibo passing in front of a magnitude 13.6 star and blocking out its light from New South Wales, Australia. Each observed the stellar occultation using high-speed video cameras attached to their 30-cm aperture telescopes at their home observatories, separated from each other by 0.7 km (0.43 mi) across (perpendicular) and 18 km (11 mi) along (parallel) the path of Arecibo's shadow across Earth. They detected a two-second-long occultation starting at 17:58 UTC, but then unexpectedly detected a secondary, shorter-duration occultation three seconds later. The observed drop in the star's brightness for both events was much greater than would be expected for a double star with one component occulted, leading Gault and Nosworthy to the conclusion that the secondary occultation was more likely caused by a natural satellite orbiting Arecibo.

Several days after the discovery of Arecibo's satellite, other occultation astronomers were alerted to follow up in another occultation event by Arecibo over North America on 9 June 2021. Richard Nolthenius and Kirk Bender, separated from each other by 8.2 km (5.1 mi) across and 8 km (5.0 mi) along Arecibo's shadow path, successfully observed the 9 June 2021 occultation from central California. As Arecibo passed in front of a magnitude 12.0 star, they detected the primary two-second-long occultation starting at 10:58 UTC and then the satellite make a secondary occultation three seconds later, confirming the existence of Arecibo's satellite. The satellite discovery and confirmation results from the May and June 2021 occultations were formally published by Central Bureau for Astronomical Telegrams on 20 June 2021. Discoverers Gault and Nosworthy recognize Arecibo's satellite as the first asteroid moon discovered by amateur astronomers, and confirmed using the occultation method.

On 30 June 2021, astronomers across the United States prepared for another occultation by Arecibo to further follow up on its satellite, but majority of them experienced technical difficulties and unfavorable weather conditions, resulting in only 5 out of 15 different sites making successful observations. Only 3 of the 5 successful sites reported positive detections (Nolthenius, Bender, and Christopher Kitting of CSU East Bay reported positives) with a single occultation; the other two had misses and did not detect the satellite. It is possible the satellite and main body were a blended image, given the close orbit later determined.

Arecibo orbits the Sun in the outer main-belt at a distance of 3.0–3.6 AU once every 5 years and 11 months (2,156 days; semi-major axis of 3.27 AU). Its orbit has an eccentricity of 0.09 and an inclination of 2° with respect to the ecliptic. Arecibo's orbit is similar to that of the large asteroid 24 Themis, which indicates that Arecibo belongs to the Themis family (602), a very large family of carbonaceous asteroids that are believed to have originated as fragments from an impact event on Themis.

Because of its low orbital inclination, Arecibo is visible along the ecliptic at apparent magnitudes 16–18. Arecibo is too faint to be seen with the naked eye, even when at its peak brightness of magnitude 16 at opposition—a telescope of at least 60 cm (24 in) in aperture size is required to see it.