"Brussels cannot force its will on Hungary," the defiant firebrand leader told his supporters in Budapest.
Although a whopping 98 percent of those who voted supported his bid to reject the proposal, turnout reached just below 44 percent of the eight-million-strong electorate, falling short of a 50-percent threshold.
The National Committee declared the referendum invalid after counting the ballots on Sunday evening.
But Orban vowed there would be "legal consequences" nonetheless, as he sought to downplay the significance of the low turnout.
A child cools off with water at a makeshift camp for migrants in Horgos, Serbia, meters away from Serbia's border with Hungary, July 27, 2016. Source: AAP
"Brussels or Budapest, that was the question, and the people said Budapest," he said.
"I will propose to change the constitution (which) shall reflect the will of the people. We will make Brussels understand that it cannot ignore the will of Hungarian voters."
Opposition figures earlier warned that any legal amendment based on the referendum result would violate the constitution.
