<p style="font-weight: 400;"><span>The widespread use of synthetic hydrophilic polymers, such as polyvinylpyrrolidone (PVP), has raised concerns about their potential effects on environmental biogeochemical processes, yet their impact on sediment ecosystems remains largely unexplored. We investigated how PVP influences methane (CH₄) production and prokaryotic community composition in freshwater sediments over a 56-day anoxic incubation. PVP exposure accelerated the onset of methanogenesis, increased maximum CH₄ production rates, and elevated maximum CH₄ concentrations. These functional changes were accompanied by shifts in bacterial communities, particularly an enrichment of fermentative Clostridia, which generate key substrates for methanogens (</span><span>H₂, acetate, and formate</span><span>). Nonetheless, archaeal communities, including methanogens, exhibited comparatively minor or transient responses. Mechanistically, enhanced CH₄ production likely resulted from a combination of increased substrate availability, altered redox microenvironments, and indirect reductions in competing electron acceptors. Our results suggest that PVP modifies sediment carbon cycling through complex microbial, biogeochemical, and physical interactions rather than direct toxicity to methanogens. These findings highlight the need to consider both chemical and physical effects of synthetic hydrophilic polymers on sediment microbial ecosystems and greenhouse gas emissions, and they underscore the importance of targeted studies to quantify these impacts in natural environments.</span>