Drawing upon social comparison theory, we investigated the influencing mechanism between relational job crafting and workplace loneliness and its boundary condition. A multiple-source, multi-wave field study was conducted with data collected from 267 employee–peer dyads in three Chinese firms. The results showed that there is an inverted U-shaped relationship between promotion-oriented relational crafting and loneliness via peer social undermining. Meanwhile, prevention-oriented relational crafting is positively related to loneliness through peer social undermining. Moreover, impression management motive moderates the inverted U-shaped relationship between promotion-oriented relational crafting and peer social undermining. Implications of these findings are discussed.