In this letter, a 2.5MHz 320-element 1.5D active matrix phased-array ultrasound transducer was designed and fabricated using high-temperature relaxor-based piezoelectric ternary Pb(In1/2Nb1/2)O3–Pb(Mg1/3Nb2/3)O3–PbTiO3 (PIN–PMN–PT) single crystals. The experiment and finite element analysis with the PZFlex package software were developed to design and optimize the 1.5D transducer. The FEM results show that the −6dB fractional bandwidth is up to 83% using two λ/3.75 matching layers. A 64×5 active element PIN–PMN–PT single-crystal phased array was fabricated and tested. The measured center frequency and −6dB fractional bandwidth were 2.5MHz and 71%, respectively. Elevation beam profiles were obtained using a hydrophone, which showed that the measured −6dB slice thickness was approximately 3mm throughout the entire range of field of interest. The PIN–PMN–PT single crystal with over 30 degree usage temperature and higher Ec field should satisfy the demands of advanced high-density ultrasound transducer applications.