Supporting Afterschool STEM Act - Requires the Director of the National Science Foundation (NSF) to establish an afterschool STEM (Science, Technology, Engineering, and Mathematics) grant program awarding competitive grants to afterschool or STEM networks to:

• develop quality standards for STEM programming in afterschool programs and provide those programs with technical assistance in implementing such standards,
• work with state education stakeholders to define and promote appropriate measurable outcomes for afterschool programs that include STEM content,
• provide technical assistance to afterschool programs to start or grow their afterschool STEM efforts and define appropriate learning outcomes for such efforts,
• coordinate professional development for afterschool program educators,
• help afterschool program providers form strategic partnerships to advance STEM learning in afterschool programs,
• create and disseminate tool kits to afterschool programs wanting to form partnerships and incorporate STEM professionals as mentors and role models, and
• provide technical assistance to federally funded STEM researchers and professionals who wish to engage with afterschool programs.

Defines: (1) an "afterschool program" as a structured program offered for elementary, middle, or secondary school students when school is not in session, such as before or after school, on the weekend, or during the summer; and (2) an "afterschool or STEM network" as a coalition that fosters partnerships and provides support to afterschool program providers and STEM education providers.

Includes as goals of the grant program: (1) to support the development and delivery of high-quality STEM education to populations underrepresented in STEM fields, and (2) to provide hands-on learning and exposure to STEM research facilities and businesses through in-person or virtual distance-learning experiences.

Requires the Director to provide each recipient of a STEM research grant who is under the Director's authority with information on opportunities to engage with students in out-of-school-time programs, such as through mentorships.