This paper reports an ultralow noise sensor consisting of a novel tantalum electrode and a low noise chopper amplifier for marine electric field measurement. The tantalum electrode is manufactured by sintering high purity tantalum powders into a tight body and forming the tantalum oxide dielectric layer by anodizing the body. Electrochemical impedance spectroscopy is used to electrically characterize the interface. Noise circuit of the chopper amplifier is modeled. Electrical properties such as source impedance, frequency response and noise were measured. The measurement results imply that the chopper amplifier has a noise of 0.42 nV/rt(Hz), tantalum electrode has a noise level equivalent to a perfect resistor of 27 ohms, and the sensor system has a noise of 0.83 nV/rt(Hz). The ultralow noise sensor could be used for marine electric field measurement with high precision.