The widespread use of thiamethoxam, a neonicotinoid pesticide, poses increasing risks to environmental and food safety, highlighting the urgent need for rapid, selective, and biomass-derived analytical platforms. In this study, fluorescent carbon dots (CDs) were synthesized from rice straw via a water-based hydrothermal approach and subsequently functionalized with a biopolymer-based molecularly imprinted polymer (MIP) for the selective detection of thiamethoxam. The synthesized CDs exhibited strong blue fluorescence with a maximum emission wavelength at 455 nm. Integration of the CDs with a thin MIP layer generated specific binding cavities complementary to thiamethoxam, significantly enhancing selectivity toward the target analyte over structurally related neonicotinoids. The resulting CD@MIP sensor demonstrated rapid and concentration-dependent fluorescence quenching, with a wide linear detection range from 1 to 500 nM and a limit of detection of 9.15 nM. An imprinting factor of 3.54 confirmed the selective recognition capability of the imprinted system. The developed sensing platform was successfully validated using real environmental water samples, achieving recovery values between 100% and 102% with relative standard deviations ranging from 0.9% to 2.5%, thereby demonstrating good accuracy and reproducibility. Overall, this work presents a biomass-derived and efficient fluorescent sensing platform for monitoring pesticide residues, with promising applications in environmental monitoring and food safety analysis.