In this study, the lipid-lowering effects of κ-carrageenans with different molecular masses (5.2, 15.5 and 36.2 kDa), prepared using a photocatalytic degradation method, were compared in a mouse model of obesity induced by a high-fat diet. Body mass, epididymal fat mass, oral glucose tolerance, four serum lipid parameters (triglycerides (TG), total cholesterol (TC), low-density lipoprotein cholesterol (LDL-C), and high-density lipoprotein cholesterol (HDL-C)), inflammatory factors (interleukin-6 (IL-6) and tumor necrosis factor-α (TNF-α)), hepatic lipid content, and pathomorphological changes in the liver, colon, and adipose tissue of mice were measured. The results indicated that all degraded κ-carrageenans exhibited lipid-lowering activity in a molecular mass-dependent manner. Among these, the 15.5 kDa κ-carrageenan demonstrated the most pronounced lipid-lowering effect. It significantly inhibited body mass gain and epididymal fat accumulation in high-fat diet-fed mice (P < 0.05) and alleviated impaired glucose tolerance, effectively regulating blood lipid levels, reducing serum TC, TG and LDL-C by 12.40%, 8.74%, and 5.02% respectively, whilst significantly increasing HDL-C levels by 11.34%. Additionally, it significantly reduced serum levels of the inflammatory cytokine IL-6 and effectively alleviated hepatic steatosis. This study revealed that the lipid-lowering activity of κ-carrageenan is closely related to its molecular mass, but does not simply increase as its molecular mass decreases. This study provides new experimental evidence for an in-depth understanding of the biological activity of κ-carrageenan, and offers new data support and theoretical references for the development of κ-carrageenan-based functional foods with specific molecular mass characteristics.