Background: Loneliness is prevalent and increases risks of disease and premature death. We aimed to investigate whether greater exposure to nature is associated with less loneliness in urban Bulgarian adults, and whether the association was modified by sociodemographic factors. Methods: The analytic sample comprised 3604 adults from a cross-sectional population-based survey conducted in 2023 in the five largest Bulgarian cities. Loneliness was self-reported with a single item on a seven-point Likert scale. Availability of nature was assessed by several GIS-derived indicators: normalised difference vegetation index (NDVI), tree cover density, urban green space, all in a 300 m buffer around home, and distance to blue space. Self-reported nature indicators included domestic garden, green space and blue space quality, green space and blue space window view, and time spent in green and blue space. Adjusted negative binomial regressions with random intercept for city district were used to assess associations with loneliness. We further checked effect modification by city type, sex, age, relationship, employment status and education. Results: Residing in areas with more urban green space, higher green space quality, and green space window view and spending more time in green and blue space were associated with lower loneliness scores. Living in low or high compared to medium level NDVI settings or in areas with higher tree cover density was associated with higher loneliness scores. Sociodemographic factors modified some of these associations. Conclusions: Our findings underscore the importance of extending and improving structured urban green spaces in Bulgarian cities.