The objective of this study was to evaluate the long-term effects of the interaction between different temperatures and reoxygenation rates on the hematological parameters, growth performance, and oxidative stress of juvenile pacu (Piaractus mesopotamicus). A total of 216 animals were used and maintained for 45 days at two temperatures (24.11 ± 0.10 and 27.08 ± 0.05ºC). They were subjected to two periods of severe hypoxia on day 15 (0.83 ± 0.07 mg O2 L⁻¹) and day 30 (0.84 ± 0.01 mg O2 L⁻¹), each lasting 8 hours, followed by different reoxygenation rates: abrupt (return to normoxic levels within 20 minutes) and gradual (increase of 1.0 mg O2 L⁻¹ per hour over 5 hours). A control treatment for each temperature was maintained under normoxia (≥ 7.0 mg O2 L⁻¹) throughout the experiment, resulting in six treatments (in triplicate). At the end of the experiment, biometric measurements (n=36 animals per treatment) were performed to assess growth performance. Subsequently, samples of blood, gills, brain, liver, and muscle were collected (n=9 animals per treatment) for hematological and oxidative stress analyses. Results demonstrated that growth was optimized at 27ºC, accompanied by higher blood glucose levels compared to juveniles kept at 24ºC (regardless of reoxygenation rate or normoxia). Abrupt and gradual reoxygenation at 27ºC led to decreased erythrocyte counts and hemoglobin levels, as well as increased mean corpuscular volume (MCV), relative to normoxia at 27ºC and the same reoxygenation treatments at 24ºC. The interaction between 24ºC with both reoxygenation rates and 27ºC with abrupt reoxygenation induced increased total antioxidant capacity against peroxyl radicals (ACAP) in liver and muscle. Furthermore, these same treatments (except abrupt reoxygenation at 24ºC) exhibited higher lipid peroxidation (LPO) levels in the analyzed tissues. No mortality was observed, indicating that the species tolerates severe hypoxia and reoxygenation. In conclusion, exposure to different temperatures and reoxygenation rates causes changes in ACAP and lipid peroxidation in juvenile pacu. However, the animals showed greater physiological and biochemical adjustment capacity when subjected to gradual reoxygenation at 27ºC.