Nearly half of girls aged 11 to 18 have suffered harassment or abuse on social media, according to a survey.
The poll found close to three-in-four (73%) have had to take action to avoid criticism online, such as blocking other users or staying off sites altogether.
Tanya Barron, chief executive of the charity Plan International UK, which commissioned the survey, said: "Girls have previously told us that they face harassment in schools and are scared everyday on the street.
"This new survey now shows that what they're experiencing in the physical world is spilling over into their digital world - and we mustn't allow this to happen."
The survey of 1,002 boys and girls in the UK aged 11 to 18 showed that 48% of female respondents had experienced some form of harassment or abuse on social media.
This could include receiving upsetting direct messages, having images shared without their consent, or feeling harassed through regular contact.
The proportion of boys reporting similar experiences was slightly lower, at 40%.
Plan International UK said the survey also revealed the lengths girls are going to in order to stop receiving abuse and criticism online.
Of the females surveyed, 73% said they had taken some form of action to avoid criticism on social media, compared with 59% of boys.
More than two-in-five girls had blocked other users (43%), a third had opted not to participate in a debate or conversation online for fear of being criticised (34%), while 13% had stopped going on social media altogether.
Ms Barron said: "What's really concerning is that our research shows girls are self-censoring on social media for fear of backlash from others."
The charity is launching the #girlsbelonghere campaign to tackle the issue.