Although loneliness has been associated with negative perceptions of social life in past research, little is known about the implications of loneliness for interpersonal perception within close relationships. The current research includes three studies (total N = 1,197) suggesting that loneliness is associated with a negative bias in perceiving relationship partners' regard and care and that this bias partially accounts for the effects of loneliness on lower relationship quality and problematic interpersonal behaviors. Loneliness was associated with perceiving family members (Study 1), friends (Studies 1 and 2), and romantic partners (Studies 1-3) as less admiring and caring, and these effects were independent of a variety of accuracy benchmarks, including partners' self-reports (Studies 1-3), reports from informants (Study 2), and objective observers' assessments of partners' responsive behavior (Study 3). Loneliness also predicted changes in perceptions of partners' regard over time (Study 3) and indirectly predicted lower relationship satisfaction, commitment, self-disclosure, and support provision through negative perceptions of relationship partners' regard and care (Studies 1-3). Studies 2 and 3 replicated these results in terms of day-to-day experiences (total daily observations = 16,064). The negative perceptions of partners' regard and care associated with loneliness predicted subsequent loneliness (Studies 2-3). Loneliness effects were statistically independent of self-esteem and attachment insecurity in all studies. Taken together, these findings suggest that, due to negative biases in perceiving relationship partners' regard and care, loneliness may compromise the quality of close relationships, motivate interpersonally problematic behaviors, and become persistent. Implications are discussed. (PsycInfo Database Record (c) 2024 APA, all rights reserved).