Betulin, a pentacyclic triterpenoid, abundantly accumulated in Inonotus obliquus (chaga mushroom), exhibits strong anti-inflammatory, antioxidant, anticancer, and wound-healing properties. However, its extraction remains challenging due to its poor solubility and thermal sensitivity. In this study, we optimized ultrasound-assisted extraction (UAE) using response surface methodology (RSM) to maximize simultaneous betulin and antioxidant compound recovery from I. obliquus. We evaluated three extraction variables (i.e., time, temperature, and ethanol concentration) using a three-factor, five-level design. All quadratic models were significant (p < 0.05), with R² values ranging from 0.83 to 0.93 and prediction errors remaining below 5 %, thereby confirming strong model reliability. Multi-response optimization using a superimposed response plot identified 92.08 % ethanol, 42.56 min, and 62.19 °C as a narrow optimal region, in which all responses simultaneously met the desired criteria. Under these conditions, extraction was predicted to yield high phenolic content (2.58 mg GAE/g DM), increased flavonoid levels (0.57 mg QE/ g DM), strong DPPH radical-scavenging activity (87.49 % DSA), and a betulin content of 2.00 mg/g DM. In contrast, low ethanol concentrations, excessive heating, or prolonged extraction times resulted in reduced yields due to the oxidative or thermal degradation of the bioactive constituents. Overall, the optimized extraction conditions emphasize the importance of controlling solvent polarity and balancing the temperature and time parameters to prevent thermolabile compound decomposition. These results provide a reproducible and eco-efficient framework for large-scale antioxidant constituent extraction from I. obliquus.