Researchers have documented associations between family relationships and a variety of well-being outcomes. Yet, sibling relationships, the longest lasting relationships in most people's lives, have received very little research attention beyond young adulthood. The goals of the current study were to: provide descriptive information about sibling relationships in later adulthood, investigate predictors of individual differences in sibling relationship quality, and examine associations among sibling relationship quality, loneliness, and well-being in later adulthood. The sample included 608 older adults (329 men, 279 women) who were 64.6 years old (SD = 4.58) on average. Participants provided self-report data about their relationships and well-being. Results showed that older adults reported high levels of sibling warmth and low levels of sibling conflict and parental favoritism. Sister-sister pairs had warmer sibling relationships than other gender-compositions. Sibling conflict and parental favoritism were positively associated with symptoms of depression, anxiety, hostility, and loneliness. Sibling warmth was negatively associated with loneliness. Loneliness partially mediated the associations between sibling relationship quality and well-being. Results from this study highlight the importance of sibling relationships in older adults' health and well-being. (PsycINFO Database Record (c) 2020 APA, all rights reserved).