The area's tremendous botanical heterogeneity and its diversity of habitats have contributed to the diversification of Montsec's valuable fauna. Species of all kinds coexist in a habitat in which the wealth of the local terrains and waters has permitted the existence and subsistence of a truly exceptional range of fauna.

Within the Montsec area, there are two zones that enjoy special protection of their fauna:

1.- The Reserva de Fauna Salvatge (Wildlife Reserve) de Sant Llorenç de Montgai
This Reserve occupies 468 hectares located in the municipalities of Camarasa and Os de Balaguer, an area that also contains the Partidor de Gerb (skimming/dividing wall) and the Embassament de Sant Llorenç (reservoir). Here, it is possible to find a number of relatively rare species of fauna, including the bearded vulture and the capercaillie.

2.- The Reserva Natural Parcial(Partial Nature Reserve) de la Noguera Ribagorçana-Mont Rebei.
This nature reserve occupies an 11 km stretch of riverside. It received its nature reserve status on account of its water predator, the otter. The scarp slopes and precipices of this mountain range provide excellent hideouts for the birds of prey that fly over Montsec both during the day and at night. These include the peregrine falcon, the vulture, and the golden eagle.

The abundance of water in this territory has permitted the existence of an important number of aquatic species of fauna: bird species include the mallard, the grebe and the grey heron. The most significant fish species include the autochthonous carp and barbel and the more recently introduced black-bass, which is found in the reservoirs.