Coordinates: 18°06′52″N, 66°10′05″W

Guaynabo (pronounced as IPA: [ˌɡwaiˈnabo]; also known as Guaynabo City or GC) is a municipality in the northern part of Puerto Rico, located in the northern coast of the island, north of Aguas Buenas; south of Cataño; east of Bayamón; and west of San Juan. Guaynabo is spread over 9 wards and Guaynabo Pueblo (the downtown area and the administrative center of the city).

Guaynabo is considered, along with its neighbors -- San Juan and the municipalities of Bayamón, Carolina, Cataño, Toa Alta, Canovanas, Caguas, Toa Baja, and Trujillo Alto -- to be one of the cities of the Área Metropolitana (Metropolitan Area) of San Juan (the largest MSA in Puerto Rico; many Puerto Ricans erroneously consider it the only metropolitan area in the Commonwealth). The municipio has a land area of 70.26 km² (27.13 sq mi) and a population of 100,053 as of the 2000 census.