A survey for the Internet Watch Foundation (IWF) suggests fewer than half of young men would contact police if they found such images online.
And 38% admitted they'd never tell their family if they stumbled across images or videos of child sexual abuse.
The IWF has an online facility to report illegal CSA images anonymously.
The Comres survey of 1,035 UK males aged 16-24 found that 44% of respondents would contact police if they inadvertently found images of child sex abuse online.
Some 54% said accidentally clicking on child sexual abuse (CSA) images and grooming was their main online worry.
Susie Hargreaves, chief executive of the IWF, said while it was encouraging that a large proportion of young men would contact the authorities, the rest needed to be told how to report images of abused children.
"We need to get that message out to more young people - and let them know that they can report these disturbing illegal images to our hotline, anonymously," she said.
Since its launch almost 21 years ago, the IWF has identified and removed more than a quarter of a million web pages showing children being sexually abused.
Three-quarters of British men aged 16-24 (75%) told the pollsters that they had not experienced any of the online incidents tested in the IWF survey in the last 12 months.
These include sexting, cyber bullying, identity theft, online grooming, exposure to images showing sexual abuse of children online, or being a victim of online shaming or revenge pornography.
Young men need to know more about how to contact the authorities if they stumble across child sexual abuse images, an internet group has said.