It is the largest single country appeal ever made by the UN, which plans to help 19 million people.
An estimated 240,000 people are facing catastrophic levels of hunger.
Meanwhile, the UN's Yemen envoy says aid workers could soon access a key granary near the port city of Hudaydah.
Martin Griffiths told the UN Security Council that government forces and Houthi rebels could start to withdraw from around the city as part of a deal reached in December which has yet to be implemented.
Hudaydah, which has been controlled by the Houthis since 2014, is the principal lifeline for two-thirds of Yemen's population. Up to 80% of the humanitarian aid, fuel and commercial goods on which they depend are delivered through the port.
Although the city has been calm recently, fighting has continued in other parts of the country, hampering the distribution of aid, the UN says.
"Some 20 million people need help securing food, including nearly 10 million who are just a step away from famine," UN emergency relief chief Mark Lowcock said.
Yemen, gripped by the world's worst humanitarian crisis, has been devastated by a conflict that escalated in early 2015, when the Houthis forced President Abdrabbuh Mansour Hadi to flee abroad.
Alarmed by the rise of a group they saw as an Iranian proxy, Saudi Arabia and eight other Arab states intervened in an attempt to restore the government.
At least 7,000 civilians have been killed and 11,100 injured in the fighting, according to the UN. Thousands more have died from preventable causes, including malnutrition and diseases.