Plastic pollution in aquatic environments has become a global concern, and agricultural plastics are one potential source. Biodegradable plastics have been developed as an alternative; however, their degradation behavior under flooded soil conditions remains unclear. Therefore, we herein investigated the degradation of poly(3-hydroxybutyrate-co-3-hydroxyhexanoate) (PHBH), a biodegradable polymer, using an in vitro system simulating paddy soil environments. Microcosm experiments were conducted under different temperatures (14, 25, and 35°C) and soil-water ratios (1:1, 1:10, 1:100, and 1:1,000) to evaluate PHBH degradation and associated gas production. The results obtained showed that temperature and the soil-water ratio both significantly affected PHBH degradation and gas production. PHBH was almost completely degraded under the flooded soil condition (soil–water ratio 1:1) at 35°C, and was accompanied by the production of large amounts of methane and carbon dioxide, suggesting the involvement of anaerobic microbial processes. Degradation and gas production were markedly reduced under lower soil-content conditions. An amplicon sequencing anal­ysis of samples showing the highest degradation revealed the higher relative abundance of several anaerobic bacteria, including Ilyobacter delafieldii, Clostridium homopropionicum, and Syntrophomonas zehnderi, than in a control sample. These results suggest the involvement of an anaerobic metabolic network in the turnover of PHBH-derived intermediate metabolites. The present results will contribute to a more detailed understanding of the environmental fate of biodegradable plastics in paddy field environments.