More than half of calls made to the ChildLine service about sexual abuse last year were from children who said they had been raped, the NSPCC says.
The charity is launching a new campaign called Don't Hide It to let children know they can get help if they have been sexually abused.
Of the 8,637 calls ChildLine took in 2005 on sexual abuse, just over half - 4,414 - were about rape.
Victims knew the reported rapist in 86% of cases.
Girls made 80% of the reports to the helpline.
Of those saying they had been raped, 842 said the perpetrator was their natural father, 421 said it was a stranger and 285 said it was a boyfriend, the NSPCC said.
Head of Child Protection Awareness at the NSPCC Chris Cloak said: "It's really worrying that half of everyone who telephones ChildLine about sexual abuse, it's a rape case. That is really alarming.
"Many people don't realise how horrendous it is.
"Children need to realise they're not alone and that talking about it does help it stop."
The Don't Hide It campaign forms part of the NSPCC's Full Stop campaign urging children to speak out on all forms of sexual abuse.
Its website, www.donthideit.com, is designed to be a safe online space where children can talk about sexual abuse and learn about stopping it.
A new film and print ads showing "false face" masks hanging in children's rooms will encourage young people to visit the site.
The masks, which look like real faces, illustrate how many children keep sexual abuse a secret.
The ads advise: "If you are being sexually abused, you don't have to hide it any more."