A high-sensitivity MEMS diaphragm hydrophone has been proposed. The designed hydrophone has a higher sensitivity in comparison with its previous counterparts. Readout electronics includes an integrated MOSFET and an external operational amplifier. An integrated ring MOSFET with a piezoelectric gate has been used as the strain to the electrical current transducer. The drain of the MOSFET has been connected to an operational amplifier that converts the transistor current to the voltage and also amplifies it. An analytical relation for the sensitivity has been derived which is in an outstanding agreement with the finite-element analysis. It has been proven that the changes in the channel length and mobility are negligible, and the transistor current is merely under the influence of the pressure-induced charges on the piezoelectric surface which directly produces the vertical electric field. It has been shown that the ring MOSFET transducer can help designing MEMS hydrophones with smaller dimensions while keeping the sensitivity as much as the larger structures.