Measuring the local pressure in blood vessels is valuable in the postoperative monitoring of aneurysms. However, implanting a conventional pressure sensor equipped with power and signal cables causes difficulties during the operative procedure and carries a risk of infection after the implantation. In this study, we developed a wireless, implantable, and flexible pressure sensor. A magnetic resonance imaging (MRI) system reads out the sensor output. The proposed wireless sensor is based on an LC resonant circuit with a spiral coil and a pressure-sensitive capacitor. The pressure-dependence of the capacitance affects the magnetic field produced by the spiral coil, changing the magnetization of the nearby sample that can be observed as a signal variation by MRI. We fabricated a prototype sensor using a capacitor with a silicone elastomer as the dielectric and a spiral coil made of gold. The maximum change in the capacitance was 8% under an external pressure of 20 kPa. A change in the thickness of the dielectric elastomer caused the capacitance to change, resulting in a signal variation detectable by MRI.