This analysis gives information about length–weight relationships (LWR) for 31 fish species from different aquatic environments in the Magdalena-Cauca River basin. Some of these species are endemic of the region and important as fishing resources. Fish were caught between 2005 and 2016, employed different fishing gears depending on the aquatic environments (portable electrofishing equipment, cast nets and gillnets). Standard length and total weight of all captured specimens were measured. New registers of LWR are presented for 26 species and new maximum values of size are given for 13 species. This study provides information for the estimation of body size models, which is important to advance our knowledge of fish and to generate recommendations that contribute to the management of these species in the aquatic systems of the Colombian Andes.