Isabel Barreto de Castro (Pontevedra, Galicia, Spain), (c. 1567 – 1612) was a Spanish sailor and traveler, one of the earliest known woman to hold the office of admiral in the history.
In the Santa Cruz Islands, she replaced Mendaña and her brother, Lorenzo Barreto, as Adelantada and Governor after their death.
Doña Isabel was honoured in Manila, and Quirós was commended for his service; with both absolved of any responsibility for the results of the expedition.
She had an uncompromising attitude and managed to maintain severe discipline of the crew of tough and adventurous men, always willing to conspire and mutiny.
She remarried to general Fernando de Castro, again crossing the Pacific Ocean to Mexico, and then settled in Buenos Aires, where they lived for several years, before returning to Peru.