A new aluminum alloy (AlSi5Mg0.5Cu0.3Ag) for semisolid die-casting applications was designed, starting from computational thermodynamics calculations by Computherm Database. The goal was to obtain a combination of good castability and proper concentration of hardening elements for strengthening precipitation treatment. The predicted thixotropic properties were verified by measuring the microstructural conventional parameters, such as globule size and shape factor, and the solidification range, by means of differential scanning calorimetry. To complete the characterization of this new alloy and to evaluate its applicability in industrial production, the shear rate and time-dependent flow behavior of the alloy in the semisolid state was studied in a Searle-type rheometer. A future aim of the present research is to try to use rheology testing as the tool to optimize the chemical composition, in order to design an alloy characterized by good mechanical performances and easy processability. Considering the strong influence of the solid fraction content on semisolid alloy viscosity, the rheology tests were interrupted after a certain time and the alloy was deeply freezed using vaporized liquid nitrogen, in order to fix the microstructure and verify the correctness of the thermodynamic simulation.