Background Although loneliness is a core interpersonal risk factor for depression, its unique contribution—independent of other co-occurring affects—remains insufficiently explored in daily life. Furthermore, according to Socioemotional Selectivity Theory, the psychological impact of social disconnection may vary depending on an individual’s Future Time Perspective (FTP), yet the moderating role of FTP in the loneliness–depression link lacks empirical evidence.Objective Using Ecological Momentary Assessment (EMA), this study aimed to disentangle the unique association between momentary loneliness and depression while controlling for 13 concurrent affects, and to examine whether FTP moderates this relationship in real-world contexts.Method Fifty-nine adults completed a 14-day EMA protocol, responding four times per day via smartphone. At each prompt, participants rated 14 negative affects and one positive affect on 0–9 visual analogue scale (VAS) scales; depression was analyzed as the outcome variable, with the remaining affects entered as predictors. Using MLM, the study examined which affects were associated with depression at the within-individual level and tested whether FTP, assessed once at baseline with a validated scale, moderated the association between loneliness and depression at the between-individual level.Results Multilevel analysis demonstrated that momentary loneliness showed a robust within-person association with depression, remaining significant even after accounting for 13 other concurrent affective states. Model comparisons confirmed substantial between-person variability in the loneliness–depression relationship, providing a statistical basis for moderation. A significant cross-level interaction revealed that FTP moderated this association. Specifically, simple slope and Johnson-Neyman analyses indicated that while a more expansive FTP was associated with lower depressive levels under conditions of low momentary loneliness, it was also linked to heightened emotional reactivity; the link between loneliness and depression was significantly more pronounced for individuals with an expansive FTP compared to those with a limited FTP.Conclusions This study suggest that depression in daily life reflects dynamic emotional processes shaped by the interplay of multiple negative and positive affects. Multilevel analyses further revealed that FTP moderated the association between loneliness and depression, such that individuals with a more expansive FTP linked to stronger positive associations between momentary loneliness and depressive affect. These findings extend socioemotional selectivity theory by suggesting that FTP may function as a context-dependent framework, conferring emotional benefits under certain conditions while amplifying vulnerability in contexts of social disconnection.