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Tropical rainforest ecosystems are important when considering the global methane (CH₄) budget and in climate change mitigation. However, there is a lack of direct and year-round observations of ecosystem-scale CH₄ fluxes from tropical rainforest ecosystems. In this study, we examined the temporal variations in CH₄ flux at the ecosystem scale and its annual budget and environmental controlling factors in a tropical rainforest of Hainan Island, China, using 3 years of continuous eddy covariance measurements from 2016 to 2018. Our results show that CH₄ uptake generally occurred in this tropical rainforest, where strong CH₄ uptake occurred in the daytime, and a weak CH₄ uptake occurred at night with a mean daily CH₄ flux of −4.5 nmol m⁻² s⁻¹. In this rainforest, the mean annual budget of CH₄ for the 3 years was −1260 mg CH₄ m⁻² year⁻¹. Furthermore, the daily averaged CH₄ flux was not distinctly different between the dry season and wet season. Sixty-nine percent of the total variance in the daily CH₄ flux could be explained by the artificial neural network (ANN) model, with a combination of air temperature (T_air), latent heat flux (LE), soil volumetric water content (VWC), atmospheric pressure (Pa), and soil temperature at −10 cm (T_soil), although the linear correlation between the daily CH₄ flux and any of these individual variables was relatively low. This indicates that CH₄ uptake in tropical rainforests is controlled by multiple environmental factors and that their relationships are nonlinear. Our findings also suggest that tropical rainforests in China acted as a CH₄ sink during 2016–2018, helping to counteract global warming.