Abstract The green macroalgal biomass corresponds to an emerging and promising biofuel feedstock. Their biological pretreatment and energetic conversion to biomethane were investigated and the enhancement of biogas production using the solid-state fermentation (SSF) as an eco-friendly innovative pretreatment of Ulva sp. was precisely assessed. Compared to conventional acid and alkali pretreatments, the highest methane potential of 153 ± 3 mL CH 4 g − 1 VS with an anaerobic biodegradability of 57% was obtained using SSF pretreatment with a locally isolated Aspergillus fumigatus SL1 strain. It was 132 ± 2 mL CH 4 g − 1 VS for raw Ulva sp. with biodegradability of 49%. Acid pretreatment with 4% HCl at 150 °C had a negative effect on Ulva sp.'s methane potential while alkali pretreatment with 4% NaOH at 20 °C showed a significant effect. The proposed SSF-based pretreatment enhanced therefore biogas production of 21% and permitted an eco-friendly valorization of large amounts of abundant macroalgae.