Loneliness in nursing homes is a critical health determinant with high prevalence rates and is associated with an increased risk of mortality and significant deterioration in physical and mental health. Studying this phenomenon is imperative given the magnitude of the problem, the vulnerability of the institutionalized population, and the potential to develop evidence-based interventions that promote dignity and quality of life. Objective: To analyze sociodemographic characteristics and the incidence of loneliness among elderly people living in Residential Structures for Older People (ERPIS) in Central Alentejo. Methodology: A quantitative, cross-sectional, and descriptive study was conducted with a convenience sample of 194 institutionalized participants aged over 65 years. Data were collected using a sociodemographic/clinical questionnaire and the UCLA-Loneliness-16 Scale to assess dimensions of affinity, social isolation, and loneliness. Results: The sample was predominantly male (60.8%) and widowed, with 95.4% reporting chronic diseases. While the most frequent response to loneliness items was “Never,” 29.4% of participants experienced significant loneliness, and 25.3% showed worrying levels of social isolation. Statistically significant differences were found in the social isolation dimension regarding gender, though not for global loneliness across other variables. Conclusion: Despite a general perception of low loneliness, a significant vulnerable subgroup affects nearly 30% of the sample. The findings underscore the importance of nursing diagnostics and the implementation of systematic screening and targeted interventions to mitigate loneliness in long-term care settings.