In recent years, plentiful data has emerged indicating the detrimental effects of loneliness on well-being. One of the challenges for researchers dealing with this issue is to find the mechanism underlying the relationship. The present study investigated 293 adults, aged 19-40, and examined whether authenticity and rumination functioned as mediators in the relationship between loneliness and well-being (and its three domainspleasure, engagement, and meaning). The results of the study confirmed the lonelinesswell-being link and, additionally, revealed potential mechanisms explaining this relationship, which were of different character in the cases of the particular domains of well-being. As it turned out, authenticity was the sole significant mediator in the relationship between loneliness and meaning, and rumination played the role of key mediator between loneliness and pleasure. Both these mediators had their share in the indirect effects of loneliness on engagement and overall well-being. The relations revealed between loneliness and authenticity are, in turn, congruent with recent conceptualizations of authenticity, which emphasize the interpersonal sources of this variable.