Response surface methodology (RSM) based on a five-level-three-factor central composite rotatable design (CCRD) was employed for optimisation of formulation for production of a soy-fortified millet-based extruded snack. Effects of amount of ingredients such as ragi (40–50%), sorghum (10–20%) and soy (5–15%) on the physical properties like bulk density, expansion ratio, water absorption index and water solubility index of snacks were investigated. Significant regression models that explained the effects of different percentages of ragi, sorghum and soy on all response variables were determined. The coefficients of determination, R2, of all the response variables were higher than 0.90. Based on the given criteria for optimisation, the basic formulation for production of millet-based extruded snack with desired sensory quality was obtained by incorporating with 42.03% ragi, 14.95% sorghum, 12.97% soy and 30% rice.