Spending time in nature has benefits for wellbeing in children, but relatively little is known about how autistic children experience nature. Framed by self-determination theory, this case study addresses this gap by exploring the experiences of 25 autistic children participating in a Forest School at their specialist school in the East of England. We used deductive reflexive thematic analysis to analyse participant observation and parent and child interview data. Our results indicated that Forest School benefited these autistic children through opportunities to play, exercise autonomy, and develop practical, motor, and social skills. However, challenges were also evident, including children absconding and conflict between peers. The success of sessions seemed contingent on adherence to routines and the influence of the adults present. Our findings supported the application of self-determination theory to Forest School to promote psychological wellbeing through autonomy, competence, and relatedness in autistic children. We discuss implications for training and practice.