Considering the health outcomes of loneliness, it is important to understand the effects of the COVID-19 pandemic for older adults to facilitate detection and intervention. The aim of this study was to examine loneliness among Spanish older adults during the first wave lockdown and associated factors, in comparison to younger adults. An online survey was completed by 3508 adults (401 aged 60 or above). Older adults felt higher social loneliness than younger adults, but lower emotional loneliness. Living alone, poor mental health, and poor healthy habits were related to higher loneliness for both age groups. The results suggest that loneliness should be an important factor in primary care, and prevention efforts should be taken, e.g., by generating open and safe community spaces and contexts for social interaction and promoting the access and ability to use technologies that maintain social connections.