This paper presents a mass sensor based on a 3-degree-of-freedom (DoF) mode localized piezoelectric bulk acoustic wave (BAW) resonator system operating in the contour extensional mode for the first time. It features a quality factor of ~4200 under atmospheric pressure at a resonance frequency of ~6.36 MHz. The mass resolution based on the amplitude ratio output readout is 1.2 pg. Compared with the state of the art, the present device achieved an enhanced performance in terms of both quality factor (~2.37 times higher) and mass resolution (~307 times higher).