Although numerous studies have investigated CH₄ emissions from river systems globally, observational data remain scarce for transboundary rivers in southwestern China. Focusing on the Yunnan section of the Irrawaddy River, this study carried out cross-seasonal sampling from October 2022 to December 2023. Dissolved CH₄ concentrations in the river surface water were measured using the headspace equilibrium technique, while diffusive CH₄ fluxes across the water–air interface were quantified using the floating chamber method. The spatiotemporal distribution patterns of CH₄ and their controlling factors were systematically analyzed. The results showed that riverine CH₄ concentrations ranged from 5.6 to 476.5 nmol/L, with a mean concentration of 64.0±98.4 nmol/L. The river was consistently supersaturated with CH₄ relative to the atmosphere (mean: 2213%±3498%; range: 162%–17024%). Diffusive CH₄ fluxes ranged from 0.004 to 1.22 mmol/(m²∙d), with a mean flux of 0.26±0.22 mmol/(m²∙d). CH₄ concentrations were higher in spring and summer, yet lower in autumn and winter. Diffusive fluxes exhibited a distinct seasonal pattern, being highest in spring and lowest in summer. Analysis of environmental drivers revealed that dissolved organic carbon (DOC), acting as a carbon substrate, was present at low concentrations, potentially limiting CH₄ production. Concurrently, suspended sediments in the water column appeared to inhibit the diffusive release of CH₄. Comparative analysis with CH₄ observations from other subtropical rivers indicated that both the concentration and diffusive flux of CH₄ in the Yunnan section of the Irrawaddy River were relatively low overall. Consequently, this river section functions as a weak source of atmospheric CH₄.