Chitosan is a biodegradable, antimicrobial, antifungal polysaccharide, thus used as an additive in different industries. Effects of chitosan supplementations on silage quality traits of alfalfa were investigated in the present study. Chitosan was supplemented into alfalfa silages at four different levels: 0% – CONT group; 0.5% – CHTS0.5 group; 1% – CHTS1.0 group; and 2% – CHTS2.0 group. Neutral detergent fibre (NDF) and neutral detergent insoluble crude protein (NDICP) values increase in CHTS 2.0 group (P < 0.05). Lactic acid and butyric acid contents decreased (P < 0.05), acetic and propionic acid contents increased with chitosan treatments. On the day of opening the silages, mould production was not encountered, and yeast production decreased in chitosan-supplemented groups. Excessive yeast production was encountered, but mould development was not seen in chitosan-supplemented groups 5 days after opening the silages. Silage pH values increased with chitosan treatments (P < 0.05). So, it can be concluded that chitosan negatively influenced fermentation quality of alfalfa silage, but reduced mould and clostridial development.