Juan Diego del Castillo (1744–1793) was a Spanish pharmacist and botanist who joined Vicente Cervantes in Mexico.
Castillo wrote Plantas descritas en el viaje de Acapulco.
In New Spain, Lacasta had been joined by a group of Spanish botanists selected by Casimiro Gómez Ortega, director of the Royal Botanical Garden of Madrid.
These included Cervantes, José Longinos Martínez, and Del Castillo.
Del Castillo left a large sum of money towards the printing of their projected book Flora Mexicana.