Our study was undertaken in order to determine the effects of preliminary hydrothermal depolymerization and enzymatic hydrolysis of macroalgae biomass originating from the Vistula Lagoon on yield of the methane fermentation process in terms of quantity and quality of produced biogas. The process of enzymatic hydrolysis was conducted with a mixture of enzymes: Cellulast 1.5 L, Novozym 188, and Hemicellulase. In turn, the process of hydrothermal depolymerization was run for 120 minutes at 200ºC under a pressure of 1.7 MPa. The processed plant substrate was next subjected to mesophilic fermentation. The application of enzymatic hydrolysis contributed to an increased quantity of and improved qualitative composition of biogas produced.