Chitosans in crosslinked form wherein the crosslink derives from the reaction of said chitosans with a crosslinker chosen from among the group of aliphatic aldehydes. Chitosan crosslinking process comprising the following stages: 1) suspend chitosan in a buffer solution having a pH that ranges between 7.5 and 9.5, preferably between 8.0 and 9.0, and more preferably between 8.1 and 8.3 (suspension 1); 2) place suspension (1) in contact with an aliphatic aldehyde as crosslinker and leave to react for 1 to 24 hours at a temperature of between 15° and 40°C (suspension 2); and possibly: 3) treat suspension (2) with an organic amine compound.