Bullying is the main concern for young people aged 12 and under who contact Childline, according to the charity.
The charity exclusively told Newsround that it gave 4,006 counselling sessions last year to children who were worried about this.
The news comes as Anti-Bullying Week starts, during which many children across the UK will be taking action to put a stop to bullying.
Head of Childline Online, Cormac Nolan, said: "Bullying can be harder to escape in the modern world as technology means hurtful comments are not always left at the school gates."
One girl told the charity that some classmates posted pictures of her online with silly tags and funny faces stuck over the top, without her knowing.
Another boy told counsellors that he was being bullied via a gaming app, with another player calling him names and making threats.
Many children said that it can be difficult to talk to someone because they don't want them to know about the reason they are being targeted or because they're worried it might make the situation worse.
But the charity explains how important it is to speak to someone if you are at all worried about bullying, how you are being treated or how you feel.
If you are worried that you or anyone that you know is being bullied, speak to an adult that you trust you about it. That might be a teacher, someone in your family or another adult that you trust, like a family friend.
You can find more help and advice on the new Own It app.
You can also call ChildLine for free on 0800 1111. There is also a special area on the Childline website with lots of help and advice.