Foster care programs at animal shelters have emerged as an important tool for promoting animal welfare and supporting shelter life-saving efforts. Preliminary evidence suggests that foster caregiving may also be health-promoting for humans. The goals of this study were to investigate the experiences of foster caregivers at animal shelters based on measures of positive and negative affect, quality of life, and grief, and to describe human-animal attachment among foster populations. Between March 2022 and 2023, 131 foster caregivers from five shelters in the United States completed three online surveys before, during and after providing foster care to a shelter animal. Positive affect decreased significantly from baseline to post-foster (F = 5.71, p<0.01), particularly among dog caregivers (F = 6.17, p<0.01). Negative affect remained unchanged (F = 0.47, p = 0.63). Foster caregivers perceived their foster animal provided companionship, affection and emotional support, although dog foster caregivers reported significantly higher emotional (U = 313.50, p<0.01) and social/physical quality of life (t = 4.42, p<0.01) than cat foster caregivers. Caregivers reported low mean avoidant and anxious attachment, suggesting they were able to develop secure bonds with their foster animals. Retention of fosters was also strong, with 86% of caregivers reporting they were likely to provide foster care in the future. Our findings suggest that fostering at animal shelters may serve as a One Health intervention to offer companionship, affection and emotional support to human caregivers while promoting animal welfare. However, these benefits did not translate to improvements in caregiver mental wellbeing, so caution should be applied when considering foster caregiving as a potential mental health promotion tool.