South Sudan and Somalia will receive £100 million each in UK aid.
International Development Secretary Priti Patel announced the new packages of UK aid for 2017/18 and called on the global community to step up their support in helping those in need.
The Department for International Development (DFID) said parts of South Sudan were now in famine, adding that in 2017 there was a credible risk of another three famines in Yemen, North East Nigeria and Somalia.
In response to famine warnings in Somalia and South Sudan, Ms Patel announced the UK support to provide food, water and emergency healthcare which is hoped will save more than a million lives.
Ms Patel said: "The world faces a series of unprecedented humanitarian crises and the real threat of famine in four countries.
"These crises are being driven by conflict and drought and we must respond accordingly.
"Our commitment to UK aid means that when people are at risk of dying from drought and disaster, we have the tools and expertise to avoid catastrophe.
"In times of crisis, the world looks to Britain not just for our work on the ground, but also for our leadership internationally.
"While we step up our support for emergency food, water and life-saving care to those in need, our message to the world is clear - we must act now to help innocent people who are starving to death."
More than six million people in Somalia have no reliable access to food and there are 360,000 acutely malnourished children, according to DFID.
The department said that all the signs were pointing to a famine as bad, or worse, than the one in 2011 which killed an estimated 260,000 people.
The UK's support to Somalia will provide, emergency food to up to one million people, nutritional support to more than 600,000 starving children and pregnant and breastfeeding women, safe drinking water for one million people, and emergency healthcare for 1.7 million people.
DFID said famine had now been declared in South Sudan and more than half the population is in desperate need.
Almost five million face the daily threat of going without enough food and water, and three million people have been forced from their homes because of violence and widespread rape, the department said.
The UK's support will provide food assistance for more than 500,000 people, nutritional support to more than 27,500 children, safe drinking water for more than 300,000 people, emergency health services for more than 100,000 people, livelihood support for more than 650,000 people, and vaccinations for more than 200,000 livestock.
This is alongside continued aid to Yemen and North East Nigeria which provided food, medical supplies, water and emergency shelter to more than two million people in 2016.