BACKGROUND: In recent years, loneliness has been recognized as a public health problem, exacerbated by the COVID-19 pandemic. This study aims to identify groups of people with different trajectories of loneliness, and to explore potential determinants (sociodemographic, social, psychological, and health-related) associated with these trajectories. METHODS: In this 12-year longitudinal study, we analyzed data on 4537 Spanish adults from a nationwide representative survey. A growth mixture modeling approach was used to identify different loneliness trajectories and logistic regressions to explore the determinants of these trajectories. RESULTS: Two trajectory classes were identified: low-stable (87.86 %) and high-fluctuating (12.14 %). Marital status, living status, migration, social isolation, depression, suicidal ideation, and cognitive complaints were identified as significant determinants of belonging to the high-fluctuating trajectory. Conversely, social support, social trust, and life satisfaction were protective factors for this trajectory. CONCLUSION: This study revealed the presence of diverse courses of loneliness (each showing some distinctive characteristics from the other), outlining some relevant implications for the assessment, prevention, and management of loneliness.