The Integrated Food Security Phase Classification (IPC) said famine was occurring in Gaza City, home to hundreds of thousands of Palestinians, and it could spread south to Deir al-Balah and Khan Younis by the end of September.
The IPC determination comes after months of warnings by aid groups that Israel's restrictions of food and other aid into Gaza, and its military offensive, were causing high levels of starvation among Palestinian civilians, particularly children.
The grim milestone - the first time the IPC has confirmed a famine in the Middle East - is sure to ramp up international pressure on Israel, which has been in a brutal war with Hamas since the militant group's October 7 attack.
Israel says it plans to soon escalate the war by seizing Gaza City and other Hamas strongholds, which experts say will exacerbate the hunger crisis.
The IPC said hunger had been driven by fighting and the blockade of aid, and magnified by widespread displacement and the collapse of food production in Gaza, pushing hunger to life-threatening levels across the entire territory after 22 months of war.
More than half a million people in Gaza, about a quarter of the population, face catastrophic levels of hunger, and many are at risk of dying from malnutrition-related causes, the IPC report says.
The IPC in July said the "worst-case scenario of famine" was unfolding in Gaza, but stopped short of an official determination.
Israeli Prime Minister Benjamin Netanyahu has denied there is hunger in Gaza, calling reports of starvation "lies" promoted by Hamas.
After the publication of images of emaciated children in Gaza and reports of hunger-related deaths, Israel announced measures to let more humanitarian aid in.
However, the UN and Palestinians in Gaza say that what aid is entering is far below what is needed.
The Israeli military agency in charge of transferring aid to the territory rejected the report as "false and biased".
The agency, known as COGAT, rejected the claim that there was famine in Gaza and said that significant steps had been taken to expand the amount of aid entering the strip in recent weeks.
In a post on social media, Israel's ministry of foreign affairs also rejected the findings, saying the IPC report was "based on Hamas lies".
It said that more than 100,000 trucks of aid have entered Gaza since the start of the war, including a massive influx in recent weeks with staple foods.
"A rapidly increasing number of people, especially young children, are dying preventable deaths from starvation and disease because Israel made starvation a core part of its campaign to control the strip," said Chris Newton, an analyst for the International Crisis Group.
Israel's plan to escalate the war in Gaza City weeks after a warning that famine was beginning there demonstrates how "intentional the famine is and how Israel wields starvation", he said.
Netanyahu says more military pressure is needed to achieve Israel's goals of freeing the hostages held by Hamas and eliminating the militant group altogether.
