Leo, who has adopted a more forceful tone recently against the direction of global leadership, also firmly repeated his opposition to increased European military spending, urging politicians instead to end the wars ravaging the globe and help migrants.
"The world is undergoing a profound spiritual and cultural crisis, which is manifested in multiple forms of violence, polarisation, and mutual distrust," the Pope said in the address, which came hours after Israel and Iran renewed their attacks on one another in the most serious test of a two-month ceasefire.
"Weapons can impose a temporary silence; but they can never build an authentic and lasting peace," he said on Monday.
Leo's speech, which was delivered in Spanish and received a seven-minute standing ovation by MPs, was a rare papal address to a national legislature and the first by a pope to Spain's parliament.
It is part of a week-long visit to the country in which the pontiff has met migrants and the homeless, and called on national leaders to stop dividing their electorates.
The Pope, whose Spain tour will culminate with the pontiff meeting migrants in the Canary Islands who braved dangerous Atlantic waters to enter Europe, said a lack of help for the world's migrants was challenging "the ethical foundation of the international order".
He said countries must look for solutions that go beyond "the mere management of flows" and should address the causes that force people to leave their countries of origin, including war, poverty and climate change.
The Pope told parliament that "the moral greatness of a nation is manifested above all in its capacity to accompany, protect, and love those lives that pass through the greatest fragility".
Leo, who issued a fervent manifesto in May urging global governments to slow down the development of AI systems, called on Monday for "rigorous ethical vigilance" over how AI was used in warfare.
He said rising European military spending, which grew in 2025 by the highest amount since the end of the Cold War amid pressure from US President Donald Trump, was troubling.
The Pope in May called European rearmament a betrayal of diplomacy.
