We report a complete investigation of the structural, electronic, vibrational, elastic and piezoelectric properties of the P2 1 2 1 2 1 orthorhombic phase in cadmium diiodate (δ-Cd(IO 3) 2) by combining experiments and first-principles based calculations. We revisited the nature of the electronic band gap and suggest an indirect band gap with a value of 4.6 eV. The infrared and Raman responses were measured and the different phonon modes assigned. To date, the δ-Cd(IO 3) 2 piezoelectric response remains unknown. We reported the different mechanisms involved in its piezoelectric response from the density functional perturbation theory. The highest value of the piezoelectric-stress and piezoelectric-strain constants in the zero Kelvin limit is predicted for e 41 =-0.27 C/m 2 and d 41 =-10.32 pC/N. These sizable values associated with the thermal stability (no phase transition up to the thermal decomposition at 550°C) and a relative large electronic band gap make δ-Cd(IO 3) 2 a potential candidate for piezoelectric applications.