Theresa May is to call for "concrete measures" to ensure girls in Commonwealth countries spend at least 12 years in education.
The prime minister will pledge £212m, which Downing Street said would allow almost one million more girls to go to school.
Too often they are forced out by "discrimination, poverty, or simply the expectations of society", she will say.
It comes ahead of the Commonwealth Heads of Government Meeting in London.
Formerly known as the British Commonwealth, the Commonwealth of Nations is a loose association of former British colonies and current dependencies, along with some countries that have no historical ties to Britain.
The CHOGM summit is held every two years.
Announcing the £212m from the foreign aid budget, she will urge Commonwealth nations to agree to the 12-year goal, calling for "concrete measures that will allow it to become a reality".
She will also focus on the fight against malaria, saying the Commonwealth has a "particular duty" to tackle the disease.
"We cannot, in good conscience, talk about the young people of the world, about securing a legacy for our children and grandchildren, without tackling a disease that, worldwide, kills one of them every two minutes," she will say.