Guasti, formerly known as South Cucamonga and Zucker, is an unincorporated community in San Bernardino County, California, United States. Guasti is 3.5 miles (6 km) east of downtown Ontario. Guasti has a post office with ZIP code 91743. The first post office in the community opened under the name Zucker in 1887 and closed in 1900. The community changed its name to Guasti after Secondo Guasti, founder of the Italian Vineyard Company, which operated in the area. The Guasti post office opened in 1910.