Heavy-oil development is becoming increasingly important because of the continuous decline in conventional-oil production. For heavy-oil reservoirs, the oil viscosity usually varies dramatically during production processes such as in thermal processes. When producing heavy oil, the high viscosity is a major impediment to recovery. Oil viscosity is often correlated directly to the reserves estimate in heavy-oil formations and can determine the success or failure of a given enhanced-oil-recovery scheme. As a result, viscosity is an important parameter for performing numerical simulation and determining the economics of a project. In this work, a simple-to-use correlation has been developed to correlate the viscosity of heavy oil to temperature and to a simple correlating parameter that can be used for heavy-oil characterization. The reported results are the product of the analysis of heavy-oil data collected from the open literature for various heavy-oil fields around the world. The tool developed in this study can be of immense practical value for petroleum engineers, providing a method for quick assessment of the viscosity of heavy oils. In particular, petroleum and production engineers would find the proposed correlation to be user-friendly, with transparent calculations involving no complex expressions. The new proposed correlation shows consistently accurate results. This consistency could not be matched by any of the widely accepted existing correlations within the investigated range. For all conditions, the new correlation provided better results than existing correlations in the literature.