The research goal is to determine the level of accuracy of Adriatic Sea renderings on selected portolan charts by using the quantitatively–analytical approach and to determine the level of portolan charts usability for navigational purposes. The manuscript is divided into two main subject matters, each divided on three and four core chapters respectively. In the first subject matter, map projection that is geometrically most similar to the renderings of Adriatic Sea on portolan charts has been determined catrometrcally. Afterwards, chronological variations of mapping accuracy, along with geometrical deviations of particular charts from their average geometry, were determined compared to that projection. Also, in this part of the research, the Adriatic Sea basin was divided regionally and functionally, which resulted in detection of mapping accuracy variations for each particular segment of those divisions. In the second subject matter, the first aspect of charts’ usability are physical dimensions of Adriatic Sea renderings on charts themselves. The second aspect is the relation between portolan charts’ counterclockwise rendering tilt and direction and value of contemporary magnetic variation. The third aspect is the relation between terrestrial navigation across the Adriatic Sea and the portolan chart accuracy, whilst the fourth aspect represents geometrical interrelation between the accuracy of portolan charts and the accuracy of textual data on navigational routes contained in selected portolans. One of the fundamental conclusions is that the renderings of Adriatic Sea basin on portolan charts show immense accuracy levels, especially if bearing-mapping accuracy of cross-basin lines is considered, and that portolan charts, at least on their segments on which Adriatic Sea is being displayed, are intentionally made with the appliance of map projection which shows the characteristics of conformal cylindrical (Mercator) map projection in its normal aspect. Also, the research results implicitly suggest that it is highly unlikely that portolan charts are genuine medieval products, but rather that they represent the copies of older charts made in certain earlier period. Fundamental arguments that support such assumption are: calculated inexistence of charts’ accuracy increase during three-century period, higher accuracy of cross-basin bearings than that of coastal bearings, and higher accuracy of cross-basin routes in portolans (in general) than that of their coastal routes.