Guaynabo, town, northeastern Puerto Rico. It is part of the metropolitan area of San Juan, lying south-southwest of the city. Founded in 1769, the town is primarily a commercial centre. The ruins of Caparra, the first Spanish settlement on Puerto Rico (1508), including the remains of the residence of explorer Juan Ponce de León, are located in the town. Fruits are grown in the surrounding rural area. Pop. (2000) 78,806; San Juan–Caguas–Guaynabo Metro Area, 2,509,007; (2010) 75,443; San Juan–Caguas–Guaynabo Metro Area, 2,478,905.