Children who watch TV for hours on end or play game consoles do worse at school, scientists claim.
They say time spent on smart devices such as mobile phones and tablets does not affect youngsters academic performance, however.
The largest study of its kind - which analysed results from 58 studies from 23 countries - involved 480,000 under 18s.
The analysts gave scores based on their results in English, maths, reading and science.
Television and video games were linked to lower composite scores, while television individually appeared to negatively affect language and mathematics skills.
Lead author Dr Mireia Adelantado-Renau, of University Jaume I in Castellon, Spain, said: "Excessive television viewing time among children has been shown to decrease attention and cognitive functioning and to increase behavioral problems and unhealthy eating habits, which may also impair academic outcomes."
He also believes video games "may trigger psychological and behavioural problems", which he says can impact their academic performance.
Dr Adelantado-Renau added: "Education and public health professionals should consider supervision and reduction to improve the academic performance of children and adolescents exposed to these activities."
He also said that his team's research was "important given the increasing time spent on screen-based activities among children and adolescents".
Next year the government is expected to issue guidelines for parents for setting maximum limits for their children's screen time in a a bid to protect their mental health.
And in February the UK's medical officers said mobile phones should be banned from the dinner table and bedtimes.
