The peninsula is covers up the Mexican states of YucatÃ n , Campeche , and Quintana Roo ; the northern part of Belize ; and Guatemala 's northern department of El PetÃ n. Mexican states situated on the isthmus to the west of the peninsula include Chiapas and Tabasco . The peninsula comprises the Mexican states of YucatÃ n , Campeche , and Quintana Roo ; the northern part of the nation of Belize ; and Guatemala 's northern El PetÃ n Department .