Waste activated sludge (WAS) has often been anaerobically digested with other types of organic waste, expecting a synergistic enhancement. In the present work, a slight amount of food waste (FW) such that the change of the substrate concentration and the C/N ratio could be neglected was added to WAS and biological methane potential tests were performed. As the amount of FW input increased, the total amount of CH4 produced and CH4 yield increased. The calculation proved that at least 30% of the increased amount of CH4 produced was derived from WAS, clearly signalling a synergistic enhancement. Measurements of the hydrolytic extracellular protease activity and ammonia concentration support the finding of synergism in that the addition of easily biodegradable organics to WAS facilitated the degradation of protein, a major constituent of WAS. This is the first report clearly revealing a synergistic effect of FW addition on the enhanced digestion of WAS.