In the United States, over 20.5 million women provide unpaid care labor to an older adult. During the last year of life, over 75% of Medicare beneficiaries depend solely on the unpaid labor of their daughters and female spouses to meet daily care needs. Significant inequities exist in the impact of unpaid eldercare on the health of U.S. adults, with women consistently experiencing more negative outcomes based on myriad metrics. In order to understand and oppose the structural forces underlying these inequities, we have developed a feminist poststructuralist theory of agency for women providing unpaid eldercare. Our theory development process included reviewing existing literature, identifying dominant and subjugated discourses, defining core concepts, and illustrating these concepts with exemplars. The two central feminist poststructuralist concepts we leverage are the discursive, intersectional construction of identity and the relational nature of agency. These conceptions form sharp contrasts with dominant biological identity essentialism and conceptions of agency as synonymous with autonomy. While individual women's agency may be constrained by the hegemonic discourse on eldercare as freely given and naturally female, relational agency to oppose external and internalized oppression can be mobilized through the emancipation of subjugated discourses.