Development of cost-effective process technologies to produce biofuels and enzymes from lignocellulosic materials has gained importance. However, few studies have considered high pressure processing (HPP) as an emerging technology for the bioconversion of lignocellulosic biomass. This study aimed to determine and optimize the effect of HPP combined with dilute acid and enzymatic saccharification on the production of fermentable sugars from hazelnut shells. Optimization via response surface methodology was carried out for acid concentration of 1 to 3% (w/v) for a pretreatment time of 10 to 30 min at a pressure range of 200 to 500 MPa. The combined HPP processes were evaluated in terms of the production of total reducing sugars. The optimized total reducing sugar production was estimated at 473.4 mg/g, with production of 88.4% fermentable sugars under 2.9% H2SO4 for 10 min at 350 MPa. The results implied a 2.4 fold increase in fermentable sugar production under the optimized conditions using combined HPP. The combined HPP process appeared to significantly lower costs due to the decreases in pressure requirement, liquid consumption, and pretreatment time.