In this paper, we describe an informal science program where specific program elements were designed to support youth development of a 21st century naturalist identity, which we call out as an important subtype of environmental and scientific identity. As part of a research-practice partnership, we conducted a case study of a teen youth program held in an urban park in order to examine how program elements support naturalist identity development. The program drew on naturalist practices to deepen youth interest and skills and provide community with human and more-than-human nature. Through participant observations, interviews, and artifact analysis, we identified three program elements that contributed to youth naturalist identity development: (1) encouraging physical interaction between humans and more-than-human nature; (2) providing meaningful opportunities for youth to be stewards of land and waters; and (3) exposing youth to possible visions of their future selves. Using ecological thinking that draws on Indigenous, post-humanist, and new materialist philosophies, we aimed to shift our gaze from individual youth experiences and towards relational processes between youth, educators, and more-than-human nature, describing how these actors work together iteratively and reciprocally to support the co-construction of naturalist youth identity development.