In this study a new mosasaur found in Colombia is given out. The fossil was extracted from Campanian rocks, North of the town of Coello, Tolima Department. It is a nearly complete articulated skeleton with soft tissue remains preserved in its cavities. It was determined as a new species of the genus Eonatator, E. coellensis, based on the systematics proposed in 2005 by Bardet and others. The anatomy of the anterior part of the skull, as well as the morphology and the inter-relationships of the bones of the pelvic girdle and limbs, constitute a new contribution to the genus definition. The Colombian specimen represents the most complete halisaurine mosasaur known so far in the world, and provides new evidences of ovoviviparity in mosasaurs.