The research was conducted via an online survey of 350 girls aged 10 t0 15 and 85 focus groups across the UK. It was sponsored by the Teenage Mental Health Foundation and Girlguiding UK.
A sixth of girls polled said they often felt angry and two thirds said they sometimes felt sad, while half said they knew someone who had become depressed.
Glossy magazines are targeting younger girls and pressurising them into losing weight and having plastic surgery, the report said.
Chief Guide Liz Burnley said: ‘Young girls today face a new generation of pressures that leave too many suffering stress, anxiety and unhappiness.
‘All of us who care about young women have a part to play in helping them find a way through these conflicting demands to build the confidence they need to be themselves.