The long-term mental and physical health impacts attributed to loneliness may result from its longitudinal association with stress. Yet, the directionality of this association remains unclear. The present study examined the bidirectional association between loneliness and perceived stress across four monthly time points. Participants (N = 1,921, Mage = 48.59, 84% women) completed measures of loneliness and perceived stress between June and September 2021. Data were analyzed using a random intercept cross-lagged panel model, which separately estimated between- and within-person effects over time. Supporting hypotheses, increases in loneliness and perceived stress, relative to expected scores, were each prospectively associated with respective increases in perceived stress and loneliness at the within-person level. In other words, loneliness and perceived stress were reciprocally related over time, even when accounting for their strong, traitlike association. Findings have key implications for methodological approaches to the study of loneliness and stress and inform mechanistic understandings of this association. (PsycInfo Database Record (c) 2026 APA, all rights reserved).