Objective: Greenspace is linked to prevention and management of anxiety, but the underlying mechanisms remain unclear. This study aims to evaluate the association between greenspace use and anxiety in Chinese young adults, focusing on the mediating role of interoception. Methods: This population-based study was conducted between September and October 2023 in four local universities in China. A total of 27,755 young adults were included. Greenspace use was assessed by self-report, anxiety by the Generalized Anxiety Disorder (GAD) assessment, and interoception by the Body Perception Questionnaire (BPQ). Generalized linear mixed models (GLMMs) and mediation models were utilized to analyze associations. Results: The mean age of participants was 19.39 ± 1.64 years, with 10,944 (39.4%) being female. A total of 10,273 (37.01%) participants suffered from anxiety. GLMMs indicated that compared to their counterparts who did not visit green spaces at all in the past four weeks, those who visited once or twice, once a week, or several times a week had lower odds of anxiety. A lower level of interoception was associated with reduced odds of anxiety. Furthermore, interoception mediated much of the association between greenspace use and anxiety. These associations were robust in a series of sensitivity analyses. Conclusions: These results suggest the crucial role of green spaces on interoception and anxiety and point to the importance of promoting student well-being through interaction with greenspace and nature. Further research is needed to better understand how interoception mediates the connection of greenspace and other aspects of the built environment with anxiety.