Free sanitary products will be offered to girls in all primary schools in England from early 2020.
It follows Chancellor Philip Hammond's announcement last month of funding for free sanitary products in secondary schools and colleges.
Amika George, founder of the #FreePeriods campaign, called the latest move "fantastic news".
Children and families minister Nadhim Zahawi said: "No-one should be held back from reaching their potential".
When Mr Hammond made the original announcement in his Spring Statement last month campaigners argued that it should also include primary schools.
"We're so glad that the government has extended this pledge to primary schools," said Ms George, 19, a student at Cambridge University.
"Period poverty should never be a barrier to education."
The next tampon
She said it meant that every student could go to school without "worrying where their next pad or tampon will come from" and could "fully participate in lessons and focus in class".
Ms George began campaigning two years ago, while still at school, after reading about period poverty in the news.
She described herself at the time as "shocked" to find out girls were missing school because of not having sanitary products.
Girlguiding says its own research has suggested almost a third of 11 to 21-year-old girls and young women have missed school or college because of their period.
Isla, also 19, and a member of Girlguiding's panel of advocates, called the figures "unacceptable", adding that "giving primary school girls access to tampons and pads will help break the taboo of periods from a young age".
A Department for Education spokesman said officials would begin talking to school and college leaders on the quantities of sanitary products likely to be needed from early next year, and how best to deliver them.
He said they were likely to be bought in bulk and distributed locally.
Mr Zahawi added: "After speaking to parents, teachers and pupils, we are now extending this to more than 20,000 primary schools so that every young person in all our schools and colleges gets the support they need."
- Last year the Scottish government announced a £5.2m scheme to provide free sanitary products in schools, colleges and universities
- And earlier in April the Welsh government announced a £2.3m grant for free sanitary products in all Welsh primary and secondary schools
- Last year a local authority in Northern Ireland announced free sanitary products in some public buildings