Social Determinants of Health (SDH) are widely recognized as fundamental drivers of health inequalities and unequal access to public health services. In universal health systems such as Brazil’s Unified Health System (Sistema Único de Saúde – SUS), formal entitlement to healthcare does not necessarily translate into equitable accessibility, as structural social inequalities continue to shape patterns of health service utilization. This analytical essay critically examines how SDH influence accessibility to public health services, focusing on structural inequalities, barriers within health systems, and policy responses aimed at promoting equity. The analysis highlights that income distribution, education, employment conditions, territorial segregation, and structural racism operate as upstream determinants that shape both health needs and access opportunities. These factors generate cumulative disadvantages that affect individuals’ ability to seek, reach, and receive appropriate healthcare, even in contexts of universal coverage. Accessibility is therefore understood as a multidimensional construct encompassing geographical, economic, organizational, informational, and cultural dimensions. Furthermore, the essay discusses how health systems reproduce or mitigate these inequalities through their organizational structures and policy designs. Persistent barriers such as bureaucratic complexity, long waiting times, fragmented care networks, and digital exclusion disproportionately affect socially vulnerable populations. While Brazil’s SUS incorporates equity-oriented principles and primary care strategies, significant challenges remain in operationalizing intersectoral actions capable of addressing the root causes of inequities. Overall, the essay argues that improving accessibility in public health systems requires moving beyond healthcare service provision and toward integrated social and policy interventions targeting the structural determinants of health.