Background <p>This study examines indigenous water governance among the Nung and Tay people in northern Vietnam, focusing on Lang Son Province, a China–Vietnam borderland adjacent to Guangxi. While earlier ethnographic studies have documented traditional irrigation techniques among Nung and Tay communities, less is known about how these systems are currently organized, negotiated, and rearticulated under changing environmental conditions and institutional frameworks.</p> Methods <p>The analysis draws on ethnographic fieldwork conducted between 2024 and 2025 in three localities of Lang Son Province, combining participant observation, in-depth interviews, and on-site surveys of irrigation infrastructure. Sixty-five Nung and Tay farmers were interviewed to document contemporary practices, decision-making processes, and local interpretations of water use and management. Ethnographic materials were thematically analyzed with attention to both technical arrangements and their social and ritual dimensions.</p> Results <p>The study shows that water management among the Nung and Tay functions as a community-based governance system centered on “phai” (weirs), “muong” (canals), “lai”, and “lin”. New fieldwork observations indicate that water-sharing practices are actively renegotiated in response to increasingly irregular rainfall, shifting cropping calendars, and the implementation of state irrigation and land-use policies. Customary rules and ritual practices continue to legitimize water allocation and communal authority, while also providing flexible frameworks for adjusting labor coordination and managing emerging conflicts. Comparison with selected Zhuang communities in Guangxi reveals shared cosmological understandings of water alongside contemporary differences in governance mechanisms and modes of institutional integration across the border. These findings challenge static representations of indigenous irrigation as a harmonious and self-regulating tradition, revealing it instead as a negotiated and uneven governance system shaped by ecological stress and institutional asymmetries.</p> Conclusion <p>By foregrounding recent ethnographic evidence, this study advances existing scholarship by demonstrating how indigenous irrigation systems among the Nung and Tay operate as adaptive, socially embedded governance arrangements rather than static technical traditions. The findings highlight the capacity of indigenous water governance to respond to climatic and policy pressures and underscore its relevance for culturally grounded and sustainable agricultural development in upland border regions.</p>