"Now more than ever there should be a commitment by everyone to not just talk but contribute food aid and allow it to reach suffering populations," he told tens of thousands of people in St Peter's Square for his weekly audience.
He said millions of people, including many children, were being "condemned to death by hunger".
Since 2013 South Sudan has been mired in civil war, which the pope called "fratricidal". President Salva Kiir, whose government declared a famine on Monday, has promised aid agencies safe access to hunger-stricken civilians.
The United Nations says it has been unable to reach some of the worst hit areas because of the insecurity.
South Sudan has been hit by the same east African drought that has pushed Somalia back to the brink of famine, six years after 260,000 people starved to death in 2011.
The UN children's agency UNICEF on Tuesday said nearly 1.4 million children were at "imminent" risk of death in famines in South Sudan, Somalia, Yemen and Nigeria.
Editor: Ade P Marboen
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