<p>Over the past few decades, China has experienced a significant increase in the number and proportion of young and middle-aged adults living alone, a trend that challenges the traditional norms of universal marriage and extended family co-residence. Despite this demographic shift, empirical evidence on the relationship between living alone and happiness among young and middle-aged adults remains limited. Drawing on six waves of cross-sectional data from the Chinese General Social Survey (CGSS, 2010, 2012, 2013, 2015, 2017, and 2018), this study employs ordered logistic regression models to examine the association between living alone and happiness, with particular attention to heterogeneity by gender, <i>hukou</i> status, marital status, education level, and birth cohort. The findings show that living alone is negatively correlated with happiness overall. Moderation analyses further indicate that the negative correlation is stronger among never-married men of both rural and urban origin (vs. their married counterparts), as well as among college-educated men of rural origin (vs. their primary-educated counterparts). Interestingly, and in contrast to the overall pattern, living alone is associated with higher levels of happiness among divorced women of urban origin (vs. their married counterparts). Finally, cohort analyses indicate that the negative correlation between living alone and happiness is stronger among never-married individuals in the 1950s–1960s cohort (vs. their married counterparts), but this pattern is not observed in other cohorts. These findings offer valuable insights for developing policies aimed at improving the well-being of individuals living alone.</p>