Although psychological benefits of natural environments continue to be established, explaining this relationship remains somewhat elusive. Ecological-self theory proposes that the well-being of humans and the well-being of nature are interwoven, whereby well-being is experienced through a spiritual interconnectedness with all things. This study aimed to (1) explore the relationships between nature connectedness, nonreligious spirituality, and six dimensions of eudaimonic well-being and (2) examine the role of spirituality as a mediator of the nature connectedness/well-being relationship. Using snowball recruitment, 155 females and 61 males aged between 18 and 77 years (M = 35.32, SD = 13.09) completed self-report questionnaires, including (1) Ryff's Scales of Psychological Well-Being, (2) the Connectedness to Nature Scale, and (3) the Mysticism Scale. All variables were significantly positively associated, and five of six mediation models were supported. Findings are consistent with ecological self theory, and implications are discussed with reference to psychological interventions, healthy living, and social planning strategies.