Loneliness is recognized as a painful experience with multiple negative consequences that has received relatively limited attention in social research. Accordingly, the present study examines feelings of loneliness, their dimensions, and the factors influencing them. The statistical population of this cross-sectional survey consisted of women and men aged 18 years and older in Tehran who had at least one living parent and did not co-reside with their parents. A total of 1000 participants were selected using a multistage cluster sampling method. The findings indicated that the mean level of social loneliness was higher than emotional loneliness. Emotional loneliness was positively associated with divorce history, a higher number of siblings, being unmarried, and conflict with parents, and negatively associated with welfare status, reciprocal emotional support between parents and adult children, and the quality of the parent–child relationship. Social loneliness showed a significant negative association with religiosity and reciprocal financial and Instrumental support between parents and adult children, and a positive association with the number of children. There are no significant differences in emotional or social loneliness between women and men, among different ethnic groups, or between employed and unemployed individuals. Emotional and social loneliness are higher among individuals with lower life satisfaction, poorer health, non-university education, and lower income. Multivariate analysis showed that health, life satisfaction, and conflict with parents were the most important predictors of emotional loneliness, whereas life satisfaction, welfare status, religiosity, reciprocal support between parents and adult children, and relationship quality played a more decisive role in explaining social loneliness. The findings indicate that loneliness is a multidimensional phenomenon, with its emotional and social dimensions being influenced by a range of individual, familial, and social factors. Therefore, interventions aimed at reducing loneliness should extend beyond individual characteristics and also pay attention to both the quantity and quality of individuals' family relationships.