This work evaluated for the first time the employment of hydrothermal alone (i.e. at 60, 80 and 100 °C) and combined ultrasonic–hydrothermal pretreatments on hazelnut shell (HS) to promote the energetic valorization of HS through anaerobic digestion. The highest cumulative biomethane yield of 137 mL CH4·g VS−1 was achieved performing biochemical methane potential tests under wet–mesophilic conditions with the hydrothermally (i.e. at 100 °C) and ultrasonically–pretreated HS. This CH4 yield was 2.3–fold higher than that obtained with the raw HS due to an enhanced hemicellulose polymerization and delignification after the sequential hydrothermal and ultrasonic pretreatment. Under the same pretreatment conditions, total volatile fatty acids peaked at 755 mg HAc L−1. The biomethane production followed the modified Gompertz model (R2 = 0.993–0.996) and a Pearson correlation test showed that it was mainly influenced by the soluble chemical oxygen demand (i.e. 0.983). A positive energy balance revealed that the produced biomethane can offset the energy needed for the pretreatment.