Informal STEM settings offer valuable opportunities for students, including neurodiverse students, to engage and participate in STEM activities. However, there is a limited information on how to best engage and include neurodiverse students in informal STEM programming. This systematic review aimed to identify the programmatic elements that facilitated the inclusion of K-12 neurodiverse STEM learners in informal STEM programs. In this systematic review, an academic and grey literature search was conducted, where the search resulted in 2632 records and 18 records were included in the systematic review. Records that met inclusion criteria were independently reviewed and assessed for quality by two reviewers using the Mixed Methods Appraisal Tool (MMAT) or a project-based MMAT. This paper describes how informal STEM programs were designed and implemented to foster the participation of neurodiverse K-12 STEM learners. A qualitative synthesis approach was used to identify the programmatic elements that fostered neurodiverse learner participation. Most of the informal STEM programming for neurodiverse youth occurred in after-school settings and with students with autism. Programmatic elements that facilitated the participation of this population in informal STEM were grouped into three categories: (1) environment/learning structure, (2) learning supports, and (3) instructional strategies and tools. Future informal STEM programming and research should expand to include the neurodiverse population in already established informal STEM programs.