Freshwater ecosystems in the Lower Mekong Basin (LMB) are increasingly affected by anthropogenic pressures such as hydropower development, altered flow regimes, and habitat degradation. While taxonomic declines are documented, the functional reshuffling of fish communities remains poorly understood. To address this gap and assess the temporal shifts in fish traits and identify those most sensitive to environmental change, we applied a trait-based approach using long-term monitoring data from four sites spanning from the upper LMB to the delta. Post-dam, we observed reduced temporal variability at most sites along with strong site-specific patterns in trait dynamics. Notably, migratory species were replaced by non-natives at the upstream site (LPB) and marine-related guilds emerged in Tonle Sap Lake (TLS) but disappeared from the delta. Community-weighted mean (CWM) values decreased for key functional groups, including rhithron residents (G1) at LPB and floodplain-related guilds at TLS. CWM-redundancy analysis identified water level, temperature, dissolved oxygen, total suspended solids (TSS), and total nitrite and nitrate (NO32) as key drivers. Upstream communities (G1 and migratory species, particularly at LPB) were correlated with high water level, dissolved oxygen, and NO32, but low temperature and TSS. Conversely, downstream communities, such as floodplain and marine related guilds, showed the opposite pattern. These findings reveal that post-dam environmental changes reshape the functional organization of the Mekong fish communities. Identifying traits sensitive to long-term environmental variation improves our understanding of how hydrological changes influence the functional composition of fish communities and can guide conservation efforts and environmental flow strategies to sustain functional fish diversity.

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