Coastal wetlands are among the most carbon-dense ecosystems on Earth, yet belowground carbon cycling remains poorly understood compared to aboveground processes. Root exudation, the release of labile organic compounds from live roots, represents a critical pathway for transferring plant-derived carbon to soils. This flux is poorly quantified because existing measurement techniques fail under flooded and tidal conditions, leaving a knowledge gap in carbon budgets. Here, we adapted and applied a sealed-cuvette system to quantify root exudation <em>in situ</em> across the two most common and widespread mangrove genera worldwide (<em>Rhizophora </em>and <em>Avicennia</em>) and across contrasting wet and dry seasons in a deltaic mangrove (Cần Giờ, Vietnam). The sealed cuvette method was successful in measuring root exudation, improving current methods for coastal wetlands. Mean root exudation rates were 126 ± 172 μg C g<sup>−1</sup> h<sup>−1</sup> for Avicennia and 68.5 ± 96.1 μg C g<sup>−1</sup> h<sup>−1</sup> for Rhizophora, with seasonal rates of 52.4 ± 67.2 μg C g<sup>−1</sup> h<sup>−1</sup> for the wet season and 135 ± 168 μg C g<sup>−1</sup> h<sup>−1</sup> for the dry season. Root exudation did not differ significantly across genera and seasons and was similar in magnitude to those of terrestrial forests. When upscaled at the ecosystem level, the root exudation is estimated to represent a considerable portion of the mangrove GPP (6.1–11.9 %). We conclude that root exudation is a non-negligible and previously unaccounted-for component of mangrove carbon budgets and highlight the need for quantification of this ‘missing carbon flux’.