There is a growing body of research that demonstrates declines in subjective well-being and increases in mental health problems among children and young people in recent decades. However, there is little comparative research examining changes in adolescents’ life satisfaction (LS) across a large number of countries, and critically, how this differs across sociodemographic groups. This study addresses this question by investigating changes in the LS of 15-year-old students between 2015 and 2018, with particular attention given to differences by gender, socio-economic status, immigrant background and urbanity. Data for this study come from the Programme for International Student Assessment (PISA). Due to the skewed nature of LS scale variables, the current study includes both mean levels of LS in a 0 to 10 scale, and the proportion of students reporting low LS (5 points or less). Linear regression models were used. Results demonstrate a global decline in mean levels of LS in 39 out of the 46 countries. In most countries, mean LS declined more among girls than among boys. Mean LS declined more, and the proportion of students reporting low LS increased more, among non-immigrant students and those of higher SES in the majority of countries. Findings regarding rural or urban communities were mixed. We advise that heterogeneity across all sociodemographic groups needs to be accounted for in public policy efforts to increase LS among young people.