<p>Fish waste (FW) is a promising substrate for anaerobic digestion due to its high biogas potential from its rich protein and fat content. However, anaerobic mono-digestion of FW can lead to problems such as fatty acid accumulation, so co-digestion with other organic wastes is often used to enhance digestion efficiency. This study evaluates the anaerobic co-digestion of FW and primary sludge (PS) through biochemical methane potential tests with varying FW: PS mixing ratios (0:100, 3:97, 7:93, 12:88, 50:50, 100:0, weight basis) to determine the optimal ratio, analyze kinetic parameters, and investigate microbial community changes. The co-digestion of FW and PS at a 12:88 mixing ratio produced the highest methane yield (459 mL/g VS_added), indicating that the co-digestion of FW and PS improves digestion efficiency. The modified Gompertz and logistic models demonstrated their capability to describe the anaerobic digestion process, with differences between measured and predicted methane yields ranging from 1.6% to 9.6% (<i>R²</i> ≥ 0.979). As the fish waste content increased, the abundance of <i>Methanospirillum</i>, which had the highest abundance, increased whereas the abundance of <i>Candidatus</i> Cloacamonas decreased. The increase in FW content has had an effect on the enrichment abundance of hydrogenotrophic methanogens, such as <i>Methanospirillum</i>. This study suggests the potential of anaerobic co-digestion of FW and PS to improve digestion efficiency while converting waste into energy for valorization.</p>