The school clothing grant will now have a minimum level of £100 in all local authorities, starting in time for the 2018-19 academic year.
The Scottish government said that about 120,000 families would benefit, with eligibility decided at a local level.
Annual costs for the change will be about £12m, with costs split between government and individual councils.
Until now it was up to councils to determine how much the grant was worth, with some offering as little as £20.
The government said that the amount would be reviewed every two years to ensure that it remained in line with the cost of living.
Analysis by Jamie McIvor, BBC Scotland education correspondent
Two years ago BBC Scotland highlighted widespread discrepancies in the value of school clothing grants across the country.
From this year they will be worth at least £100.
Individual local authorities would be able to offer an even greater grant if they want to.
They will still be able to decide exactly who should be eligible.
The cost of school clothing can however vary too so, in that sense, the actual practical value of a £100 grant could vary.
For instance, some schools or councils may have deals with particular suppliers which help bring down the cost.
But there are also signs of just how hard it is for some families to pay for school clothes.
For instance, there are "uniform bank" - schemes which allow parents to access second-hand clothes that are no longer needed, just as in some families clothes might be passed down from an older to a younger sibling.
However, today's announcement is an important move which will help ensure greater consistency in the help available to low income families across the country.
Education Secretary John Swinney said: "Every child in Scotland should be able to attend school feeling comfortable, confident and ready to learn.
"We know that school uniforms can be a considerable cost for families which is why we have worked in partnership with Cosla to introduce a new national minimum school clothing grant.
"It will help relieve pressure on families, reduce costs of living and remove the stress and stigma which can often be associated with struggling to afford essential school items."
Stephen McCabe, Cosla's spokesperson for children and young people, described the announcement as a "step in the right direction".
He said: "Access to decent clothing is an important part of ensuring children have the best chance of getting the most from their education.
"Therefore it is only right that we do the best we can to create as much of a level playing field as we can in terms of school uniform."
Support for parents
John Dickie, director of the Child Poverty Action Group (CPAG) in Scotland, said the announcement was "great news" for families already struggling to pay for essentials.
He said: "£100 is now the absolute minimum grant that must be offered to families struggling on low incomes.
"Grants are already higher in some areas, and the actual cost of kitting out a child for school was recently calculated as £129.50.
"But this new £100 minimum provides a very welcome basic level of consistent support for parents, wherever they live in Scotland."