<p>Despite growing research on gut microbiota in wild primates, seasonal functional dynamics of the gut microbiota in this endangered folivorous species remain poorly understood. This study investigates the seasonal variations in the gut microbiota of François’ langur (<i>Trachypithecus francoisi</i>) and their implications for dietary adaptation and conservation. Using shotgun metagenomic sequencing, fecal samples were collected across four seasons within the Mayanghe National Nature Reserve in China (<i>n</i> = 24). The study identified significant seasonal shifts in microbial diversity and composition. While alpha diversity metrics reflecting community evenness (Shannon and Simpson equivalents) remained stable (<i>P</i>_<i>adj</i> &gt; 0.05), species richness (Hill number, q = 0) was significantly lower in Fall compared to Spring and Winter (<i>P</i>_<i>adj</i> = 0.013). Results revealed that dominant phyla included Bacillota and Bacteroidota, with a significant enrichment of <i>Faecalibacterium</i> during Fall. Functional analysis showed a predominance of carbohydrate metabolism, which remained stable at broad metabolic levels; however, fine-scale functional units (KOs and CAZy families) exhibited distinct seasonal signatures. A moderate correlation between taxonomic and functional profiles (Mantel <i>r</i> = 0.43, <i>P</i> = 0.001) suggests a partial decoupling. These findings highlight the ecological plasticity of the gut microbiota and underscore how taxonomic flexibility enables functional homeostasis, aiding the physiological resilience of endangered primates in fluctuating environments.</p>