The proportion of Black, Latinx, and American Native individuals annually earning science, technology, engineering, and mathematics (STEM) doctoral degrees in the United States (US) has been enduringly and inequitably low for decades compared to their White peers. Despite the intransigent connection between race and STEM doctoral outcomes, US STEM education policy documents typically fail to identify racism as influencing this racial inequity. This paper presents critical capital theory (CCT)—an integration of critical race theory, forms of capital, and fictive kinship—to give racism full explanatory power within the context of US STEM doctoral outcomes. CCT proposes that access to large and affluent social networks containing supportive individuals who have knowledge of how to successfully navigate institutions of power is currently core to STEM doctoral success. This access reinforces STEM identity and belonging, but has been and continues to be primarily preserved for White students via an ever‐evolving system of racism. CCT proposes systems supporting STEM faculty’s consistent provision of high‐quality mentorship experiences for all their students—coupled with accountability for providing this mentorship—would result in more equitable STEM doctoral outcomes.