Background/Objectives: Obesity and nonalcoholic fatty liver disease (NAFLD) reduce life expectancy; nonoperative interventions show poor results. Individually, chitosan, acetic acid (AA), and sepiolite clay attenuate high-fat diet-induced obesity (DIO) via reduced energy digestibility and increased energy expenditure. We hypothesized that a chitosan–sepiolite biocomposite suspended in AA would attenuate DIO and NAFLD to a greater extent than AA alone via its more substantial adsorption of nonpolar molecules; Methods: We tested this dietary supplement in C57BL/6J mice fed a high-fat diet, compared to supplementation of a bile-acid sequestrant (cholestyramine) and standalone AA; Results: Biocomposite supplementation attenuated DIO rate and NAFLD progression, whereas standalone AA showed mild attenuation of DIO gain and did not prevent HFD-induced hepatic fat accumulation. Biocomposite intake was accompanied by a decreasing digestibility trend counterbalanced by increased intake; hence, it did not affect energy absorption; Conclusions: Therefore, DIO attenuation was suggested to be related to higher energy expenditure, a phenomenon not found with AA alone, as supported by calculated energy expenditure using the energy-balance method. These results support further investigation of the biocomposite’s efficacy in attenuating obesity and NAFLD, specifically when applied with a restricted diet. Future studies are needed to determine this biocomposite's safety, mechanism of action, and efficacy compared to its components given separately or combined with other ingredients.