This report presents new documentation of the external canal in the Late Postclassic site of Tetzcotzinco in the municipality of Texcoco, Mexico. This structure was previously considered a waterwork separate from the monumental water-management system discovered in the central part of the site. However, reanalysis of the course of this canal allowed us to reassess its function and revise the existing Tetzcotzinco maps. We propose that this structure formed part of the main water-management system of the site.