Late childhood and adolescence is a critical time for social and emotional development. Over the past two decades, this life stage has been hugely affected by the almost universal adoption of the internet as a source of information, communication, and entertainment. We use a large representative sample of over 6300 children in England over the period 2012–2017, to estimate the effect of neighbourhood broadband speed, as a proxy for internet use, on a number of wellbeing outcomes, which reflect how these children feel about different aspects of their life. We find that internet use is negatively associated with wellbeing across a number of domains. The strongest effect is for how children feel about their appearance, and the effects are worse for girls than boys. We test a number of potential causal mechanisms, and find support both for the ‘crowding out’ hypothesis, whereby internet use reduces the time spent on other beneficial activities, and for the adverse effect of social media use. Our evidence adds weight to the already strident calls for interventions that can reduce the adverse effects of internet use on children’s emotional health.