The Portuguese king who rejected Columbus's proposal was John II. Around 1484, Columbus asked Portugal's monarch to support a westward voyage to Asia. John II referred the plan to his maritime advisers, who rejected it because Columbus had badly underestimated the Atlantic distance.
Columbus calculated that the voyage from Europe to Asia would be only about 2,400 nautical miles. The advisers correctly judged that the distance was much greater. Their decision reflected Portugal's established strategy of reaching Asia by sailing around Africa, a route strengthened by later discoveries along the African coast.
John II did not simply dismiss Columbus personally. He allowed the proposal to be examined and later granted Columbus another audience in 1488, but the Portuguese court still did not finance the expedition. Columbus then turned to Ferdinand II of Aragon and Isabella I of Castile, whose court eventually approved his plan.
A common mix-up is confusing John II with Manuel I, who became king of Portugal in 1495, after Columbus's first voyage. John II was the monarch involved when Columbus initially sought Portuguese backing.