The study found a “continuous steep increase” between 2002 and 2014 in the proportion of children and young people using technology for two hours or more each weekday for things like social media, surfing the internet and homework.
While use increased for both sexes, it more than tripled for girls aged 15 and over during this period, with experts blaming the rise of social media.
The WHO, together with experts from the Health Behaviour in School-age Children study, sent questionnaires to more than 200,000 children in schools in 42 countries.
A breakdown by age showed children as young as 11 spending a large chunk of time online.
When it came to using computers, tablets or smartphones just for games, between a third and two-thirds of children were spending two or more hours every weekday on them.
Lead author Dr Jo Inchley, from the University of St Andrews, said the rise in social media was having an impact on young people.
“We know that a positive impact of social media is social connectedness and the sense of interaction,” she said.
“But we also know there are risks, such as cyber bullying and impact on mental health, as well as things like missing out on sleep.
“Also, there are longer-term impacts on physical health from being sedentary.” She said these risks included cardiovascular disease, obesity and Type 2 diabetes.
She added: “One of the main challenges for us is that this kind of activity (social media and computer use) is so much part of young people’s lives these days, how do we manage this and the health risks associated with it?
“It’s about reducing time being spent sedentary, and ensuring that children still have opportunity to be active. We really need to start addressing these challenges now.”
The report also found that while TV-watching is declining, only a minority of youngsters watch less than two hours a day.
Dr Inchley said: “We need to find ways to make young people more active.
“Maybe they are getting lifts in the car, or are not out playing in the streets any more in the way they used to.”
She said having two working parents could also affect exercise levels, with some children at after-school clubs instead of being outside.
The report is being presented at the European Congress on Obesity in Porto, Portugal.