ChildLine says the number of children contacting them about cyberbullying has almost doubled in the past year.
It saw 4,507 cases of cyberbullying in 2012-13, up from 2,410 in 2011-12.
"It used to be the case that if they went to the bedroom and shut the door, they were safe," said Claire Lilley from the NSPCC.
"Now children can be reached 24 hours a day through smartphones, tablets and devices which are not necessarily in the family room."
The charity also reports a big increase in racist bullying.
ChildLine works with online chat, email and phone.
In 2012-13, the charity helped 278,886 children and teenagers.