Previous research has reported an association between life satisfaction, self-esteem, and volunteering. However, it is unclear whether self-esteem is associated with life satisfaction in older adults who are already engaged in volunteering. Therefore, the present study aimed to investigate the association between life satisfaction and self-esteem in older adults who were formally volunteering at a non-governmental organization in Taiwan. A cross-sectional study was conducted on 186 formal volunteers aged ≥ 65 years who were recruited from the Keelung chapter of the Buddhist Compassion Relief Tzu Chi Foundation in Taiwan. A hierarchical stepwise linear regression was used to examine the association between scores on the Satisfaction With Life Scale (SWLS) with the Rosenberg Self-Esteem Scale (RSES) and the Hedonic and Eudaimonic Motives for Activities-Revised (HEMA-R) scale. The results showed that SWLS was significantly associated with RSES score (standardized beta (std. β) = 0.199, p = 0.003), the eudaimonic subscale score of the HEMA-R (std. β = 0.353, p < 0.001), a vegetarian diet (std. β = 0.143, p = 0.027), and volunteering for five days or more a week (std. β = 0.161, p = 0.011). In conclusion, improving self-esteem and promoting eudaimonic motives in older adults who are formally volunteering could be effective strategies for enhancing their levels of life satisfaction.