Stability of a dental implant reflects quality of osseointegration between the implant and its surrounding bone. While many methods have been proposed to characterize implant stability, angular stiffness at the neck of the implant has been proven to be a rigorous and accurate measure. Nevertheless, fast and reliable measurements of the angular stiffness in a clinical environment are not yet available. This article is to demonstrate a novel stability diagnostic device that can measure the angular stiffness accurately in clinical environments. The device consists of a sensing unit, a controller unit, and a mobile app. In the sensing unit, a coupler attaches a buzzer motor and a tiny accelerometer to an abutment of an implant, whose angular stiffness is to be measured. The buzzer vibrates at a frequency below the resonance frequency of the implant–bone–abutment system. Meanwhile, the accelerometer measures the abutment's vibration. The controller unit powers the buzzer, reads the accelerometer data, and transmits the data to the mobile app. The mobile app postprocesses the data and extracts the angular stiffness through use of a finite element model and a nonlinear regression algorithm. The extracted angular stiffness is compared with a calibrated angular stiffness, which is obtained independently via a force hammer and a laser Doppler vibrometer. The comparison shows reasonable agreement, with the difference being in the range of 4–20%.