Francisco Martínez Portusach (1864–1919) was a Spanish merchant and whaler who was briefly the Governor of Guam, before he was deposed.

After the death of his father, he fell out of favor with his elder brother and moved to Chicago, Illinois, in 1885 and became a United States citizen in 1888.

The existing Spanish garrison, including Governor Juan Marina, were taken as prisoners of war to Manila.

Captain Henry Glass, the commander of the battle appointed Portusach as the acting-Governor of Guam, to serve until a formal military garrison could be sent.

With Glass and the United States Navy departed, Portusach was unable to solidify his position as governor and he was quickly overthrown by José Sisto, a Filipino Spaniard and the island Treasurer under Spanish rule.