Guacio is a barrio in the municipality of San Sebastián, Puerto Rico. Its population in 2010 was 640.
Puerto Rico was ceded by Spain in the aftermath of the Spanish–American War under the terms of the Treaty of Paris of 1898 and became an unincorporated territory of the United States. In 1899, the United States Department of War conducted a census of Puerto Rico finding that the population of Guacio barrio was 562.
Barrios (which are roughly comparable to minor civil divisions) in turn are further subdivided into smaller local populated place areas/units called sectores (sectors in English). The types of sectores may vary, from normally sector to urbanización to reparto to barriada to residencial, among others.The following sectors are in Guacio barrio:Carretera 119, Carretera 424, Carretera 433, Parcelas El Guacio, Parcelas Marco Antonio, Sector Boquerón, Sector Campamento Boys Scouts, Sector Domenech, Sector Inglés, Sector Lulo Rivera, and Sector Tosquero.