A novel, wireless, passive, extra-arterial implantable blood pressure monitoring system was designed, fabricated, and tested. The measurement system is composed of an implantable sensor in-vivo and a readout system in-vitro. It was designed to balance the measurement accuracy and arterial invasiveness in continuous blood pressure monitoring. The passive LC resonant circuit was formed by a variable parallel-plate capacitance and a planar spiral inductor. It was fabricated and implanted in the necks of the rats to detect blood pressure changes. The variation of blood pressure was transduced into a change of the capacitance value of the variable parallel-plate capacitor, which caused a variation of the resonant frequency of the LC circuit. A readout probe with inductor coil was then used to measure the change of the resonant frequency of the LC circuit, and therefore indirectly measure the variation of blood pressure wirelessly. By measuring the resonant frequency with electromagnetic coupling of the implanted LC resonant sensor and the external inductor coil, a sensitivity of 8.53 kHz/mmHg for blood pressure measurement had been successfully achieved. The sensing distance is longer than 22 mm, which is enough for implementation of the wireless measurement. The overall size of the implantable sensor is 5 mm × 1.1 mm. The wireless implantable blood pressure sensor may be used in biomedical research for continuous, accurate and arterial trauma free blood pressure monitoring.