The United Nations agency previously said sugar should make up 10% of total energy intake per day for adults and children.
But they've now added to that advice, saying a further reduction to below 5% or roughly 25g (6 teaspoons) per day would "provide additional health benefits."
“We have solid evidence that keeping intake of free sugars to less than 10% of total energy intake reduces the risk of overweight, obesity and tooth decay,” Dr Francesco Branca, Director of WHO’s Department of Nutrition for Health and Development said in a statement.
“Making policy changes to support this will be key if countries are to live up to their commitments to reduce the burden of noncommunicable diseases.”
The WHO guidelines do not refer to sugars that occur naturally in fruit, vegetables or milk.
Instead, they aim to address "hidden sugars" that appear in processed food and drink. To find out what some of the worst offenders are, check out the slideshow below:
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But not everyone agrees with the WHO's updated guidelines.
Tom Sanders, professor emeritus of nutrition and dietetics at King's College London, said that although the 10% target is easily achievable, halving this would be much tougher.
"There is currently no evidence supporting a recommended intake lower than 10% for obesity prevention, either from observational studies or randomised control trials," he said.
"In the UK average intakes of free sugars are about 11-12% of the energy in adults but higher in teenagers where they are closer to 18% of the energy.
"The target of 10% can easily met be avoiding sugar sweetened beverages and getting fluid intakes preferably from water or sugar-free beverages."
He added that the recommendation of lowering it to 5% is "much harder to meet because it would involve not eating cakes, biscuits, confectionery and all sugar sweetened beverages including fruit juice".
Dr Alison Tedstone, chief nutritionist at Public Health England (PHE), has welcomed the updated guidelines, however.
"As a nation our diet contains too much sugar, especially for children and teenagers who are consuming 50% more sugar than the maximum recommended amount," she said.
"Too many sugary foods and drinks not only cause tooth decay but also mean excess calories which lead to being overweight or obese, increasing the risk of heart disease, type 2 diabetes and some cancers."