A geometrical analysis was performed using CAD (Computer Aided Design) tools on the plans of the nine Tholos Tombs of Mycenae and of the “Treasury of Atreus” in particular. Dedicated parameters were established in order to classify the main common geometrical features of the tombs. The analyses were based on a comparison between the geometrical proportions found on the plans and those of the Squaring Triads. It appears that Mycenaean architects made use of both Perfect (Pythagorean) and Quasi-Perfect combinations of integers. The Treasury of Atreus stands out by exhibiting all the major geometric proportions identifiable with those belonging to a series of Pythagorean Triads reported by Diophantus and known to the Mesopotamians. The unit of length for the Atreus Tholos Tomb coincides with the Lagash Gudea cubit of 0.496 m.