BACKGROUND AND OBJECTIVES: The current coronavirus disease 2019 (COVID-19) pandemic and social distancing measures are an extreme stressor that might result in negative emotional experiences and feelings of loneliness. However, it is possible that social relationships might have a protective effect. In the present study, we examine how the COVID-19 pandemic affected older adults' well-being and loneliness, and the role of structural and functional characteristics of social relationships. RESEARCH DESIGN AND METHODS: We use data from 99 older adults in Switzerland who participated (a) in a 3-week microlongitudinal study on social relationships and well-being in 2019 and (b) in a weekly online survey during 4 weeks of the COVID-19 lockdown. RESULTS: Our findings show that the global pandemic had substantial adverse effects on older adults' emotional well-being and loneliness. In addition, aspects of social relationships were related to loneliness both before and during the pandemic. Only one functional feature of social relationships (satisfaction with communication during the pandemic) buffered adverse effects of the major stressful event. DISCUSSION AND IMPLICATIONS: Although the social distancing measures during COVID-19 presented a major stressor for older adults' well-being and loneliness, being able to maintain social communication to a satisfactory level during that time reduced this effect. Therefore, enabling older adults to stay in touch with their social circle based on their personal preferences might reduce the impact that any future lockdown might have on their well-being.