A state of famine has been declared in the countries, with conflicts in both of them making the problem worse.
Save the Children reported on Wednesday that Somalia alone hasn't had rainfall across the country for two years. It says it's the worst drought the east African country has had since 1950.
Between the two countries, the Department for International Development says around 11 million people don't have regular access to food. The UN says £1.29 billion is needed to stabilise the situation.
The UK's aid to Somalia will provide:
Food assistance for one million including 600,000 children,
Drinking water for one million people
Emergency healthcare for 1.7 million people.
And in South Sudan it will enable:
Food assistance for 500,000 including 27,500 children
Drinking water for 300,000 people,
Emergency health services for 100,000 people,
Livelihood support for more than 650,000 people
Vaccinations for more than 200,000 livestock.
Nick Guttmann, head of the humanitarian division at Christian Aid, told Premier: "I think it's great that the British government has responded with this contribution. It's absolutely needed.
"I would always say that the British government should always give more, but coming out now and providing in response to the famine that's been declared... it's very welcome."
International Development Secretary Priti Patel said: "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 lifesaving 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."
Listen to Premier's Aaron James speaking to Nick Guttmann:
