Currently, only 20 fish species have been recorded from the Casanare River, which is one of the main tributaries of the Meta River drainage. To characterize the fish assemblages of this river, samplings were carried out at 35 collecting stations in low and high water seasons of 2015. The results obtained, raised the number of fish species of the Casanare River drainage to 180, of which 41 are exclusive to the piedmont, 83 to the savannah and 56 are shared between both units; 19 species are endemic, 11 are migratory, and three are classified in one of the national threat categories. Three species were found to be undescribed and one represents a new record for the Orinoco River Basin in Colombia. This study fills the lack of information for the Casanare River drainage, highlights its importance as a potential area for biodiversity conservation and facilitates the decision-making process related to the management planning and conservation of the hydrographic basin.