Chickpea (Cicer arietinum L.) is one of the most widely consumed grain legumes worldwide, and its protein isolates are increasingly used as ingredients in plant-based foods. This study assessed the nutritional quality of a chickpea protein isolate produced from locally grown seeds by alkaline solubilization and isoelectric precipitation and evaluated it in vivo against casein under a high-fat dietary challenge. In total, 30 male Wistar rats were allocated to three weight-matched groups for 28 days: control (20% casein), high-fat diet (HFD, 20% casein plus 20% sheep fat), and HFD with casein replaced by 20% chickpea protein isolate (HFD-CP). No indispensable amino acid determined in the isolate (tryptophan was not analyzed) fell below the FAO/WHO reference pattern. Compared with HFD, HFD-CP lowered body weight gain, adiposity, serum leptin (−29%), total cholesterol (−23%), LDL-cholesterol (−36%), hepatic triglycerides (−26%), hepatic cholesterol (−29%), and serum and hepatic thiobarbituric acid reactive substances (−29% and −35%) and increased superoxide dismutase, catalase, lecithin-cholesterol acyltransferase (+37%), and paraoxonase-1 (+33%) activities (p < 0.05), with no significant difference in energy intake between the high-fat groups. As an ingredient, the isolate therefore retains a high nutritional quality when substituted for an animal protein, supporting its use in the formulation of plant-based foods, although the present design does not allow the observed effects to be assigned to its protein fraction alone.