This study aimed to investigate the effects of oral administration of Platonia insignis Mart. (“bacuri”) seed butter (BSB) on oxidative stress and diabetes mellitus-related parameters in streptozotocin-induced (STZ) diabetic rats. Diabetes mellitus was induced in female Wistar rats (180–250 g) by the intraperitoneal administration of STZ (45 mg/kg, b.w). BSB (25, 50, and 100 mg/kg) was administered to animals for four weeks. The effect on weight gain, food intake, blood glucose, glycated hemoglobin, hepatic transaminases, plasma and liver TBARS and MPO activity, erythrocyte SOD activity, non-protein sulfhydryl groups (SH-NP), and histopathology of the liver tissue was investigated. BSB at the dose of 100 mg/kg had a positive effect on the reduction in glycated hemoglobin percentage and increased albumin concentration, as well as decreased ALT and AST levels and increased SH-NP liver levels in treated animals compared to normal control rats. Moreover, BSB had no effects on weight gain, food intake, and fasting glucose. Thus, the BSB presented marked properties in improvement of hepatic antioxidant defenses, which demonstrates BSB as a potential hepatoprotective agent in metabolic disorders.