21st Century STEM for Underrepresented Students Act - Requires the Director of the National Science Foundation (NSF) to provide merit-reviewed, competitive grants for research on programming that engages underrepresented students in grades kindergarten through 8 in STEM (science, technology, engineering, and mathematics) in order to prepare these groups to pursue undergraduate and graduate degrees or careers in STEM.

Requires such grants to be used toward research to advance the engagement of these students in STEM through providing before-school, after-school, out-of-school, or summer activities designed to encourage interest, engagement, and skills development of underrepresented students in STEM.

Requires the Director to give priority to applicants which, for the purpose of grant activity, include or partner with a nonprofit, nongovernmental organization that has extensive experience and expertise in increasing the participation of underrepresented students in STEM.