Aim <p>Intensive use of social media and poor mental health among young adults have become a topic of growing concern in many countries. Recent research assumed that addictive social media use (SMU) could contribute to a decrease of mental health. The present cross-national study investigated the mechanisms that could explain these findings in Germany and Israel.</p> Subject and methods <p>Data of young students in Germany (<i>N</i> = 238) and Israel (<i>N</i> = 126) on addictive SMU, loneliness, and mental health (operationalized by depressive symptoms and positive mental health, PMH) were collected via online surveys.</p> Results <p>In both countries, we found the same result pattern. Addictive SMU was significantly positively associated with loneliness and depressive symptoms; PMH was significantly negatively linked to addictive SMU and loneliness. Moreover, loneliness mediated the association between addictive SMU and depressive symptoms, as well as between addictive SMU and PMH.</p> Conclusions <p>The current results reveal that the higher addictive SMU, the higher loneliness and the higher depressive symptoms or the lower PMH. The effects were found in young people in Germany and Israel even though they live under varying degrees of exposure to global challenges such as military conflicts. Potential ways how to reduce their loneliness and to protect their mental health are discussed.</p>