The shedding of leaves by Posidonia oceanica (P. oceanica) in autumn results in the accumulation of shoreline debris, contributing to significant economic, social, and environmental problems. Due to the lack of alternative solutions, this waste biomass is disposed of in landfills, incurring an economic cost for the disposal process. In the context of the circular economy, anaerobic digestion (AD) can serve as a highly efficient biological alternative for treating and valorizing wastes with a high organic load. The aim of this research was to comparatively evaluate the performance and kinetics of the AD of ashore P. oceanica biomass and its anaerobic co-digestion (co-AD) with different nitrogen-rich co-substrates. To evaluate the effect of the nitrogen source in the co-AD system, peptone, casein, synthetic casein, urea, and the microalgae Raphidocelis subcapitata were used as co-substrates in biomethanization tests at a mesophilic temperature (35 ± 2 °C). The lowest methane yield was achieved for the sole AD of P. oceanica (79 ± 3 NL CH₄ kg⁻¹ VS), while the highest yields were found for the three co-ADs of P. oceanica with proteins (i.e., peptone, casein, and synthetic casein), showing no significant differences among them (380 ± 30 − 420 ± 30 NL CH₄ kg⁻¹ VS). Additionally, the first-order kinetics and the transference function model were proven and allowed for adequately fitting the experimental results of methane production with time.