Harm-advocating online content includes pro-eating disorder, pro-self-harm, pro-suicide, and the positive portrayal of the deaths of real people (snuff or death sites). This material is often user-generated and easily accessible for an average online user, therefore offering a potential source of risk for many Internet users. This cross-sectional study examined the association between exposure to harm-advocating online content and users’ subjective well-being (SWB) among American (n = 1032) and Finnish (n = 555) young people aged 15–30. Exposure to different types of online harm-advocating content was prevalent in both countries. Lower SWB was associated with exposure to this material even when controlling for social networking site (SNS) activity and online and offline victimization. In the US sample, seeing death sites was not associated with SWB, but seeing other harm-advocating sites was. In both countries, those with high SNS activity were more likely to be exposed to online harm-advocating material. These results from two advanced information societies underline the importance of recognizing the existence of harmful online communities. These communities are grounded on social interaction that might involve risks for the well-being of adolescents and young adults.