Background: Environmental volunteering can increase well-being, but environmental volunteer well-being has rarely been compared to participant well-being associated with other types of volunteering or nature-based activities. This paper aims to use a multidimensional approach to well-being to explore the immediately experienced and later remembered well-being of environmental volunteers and to compare this to the increased well-being of participants in other types of nature-based activities and volunteering. Furthermore, it aims to compare volunteer managers’ perceptions of their volunteers’ well-being with the self-reported well-being of the volunteers. Methods: Onsite surveys were conducted of practical conservation and biodiversity monitoring volunteers, as well as their control groups (walkers and fieldwork students, respectively), to measure general well-being before their nature-based activity and activity-related well-being immediately after their activity. Online surveys of current, former and potential volunteers and volunteer managers measured remembered volunteering-related well-being and managers’ perceptions of their volunteers’ well-being. Data were analysed based on Seligman’s multidimensional PERMA (‘positive emotion’, ‘engagement’, ‘positive relationship’, ‘meaning’, ‘achievement’) model of well-being. Factor analysis recovered three of the five PERMA elements, ‘engagement’, ‘relationship’ and ‘meaning’, as well as ‘negative emotion’ and ‘health’ as factors. Results: Environmental volunteering significantly improved positive elements and significantly decreased negative elements of participants’ immediate well-being, and it did so more than walking or student fieldwork. Even remembering their volunteering up to six months later, volunteers rated their volunteering-related well-being higher than volunteers rated their well-being generally in life. However, volunteering was not found to have an effect on overall mean well-being generally in life. Volunteer managers did not perceive the significant increase in well-being that volunteers reported. Conclusions: This study showed how environmental volunteering immediately improved participants’ well-being, even more than other nature-based activities. It highlights the benefit of regarding well-being as a multidimensional construct to more systematically understand, support and enhance volunteer well-being.