This work introduces a capacitive micromachined ultrasonic transducer (CMUT) with a reduced-gap architecture implemented in the PolyMUMPs technology. The proposed structure enables the realization of high-frequency CMUTs, for the first time in a commercial surface micromachined technology, suitable for in-air ranging applications, as well as immersion applications such as non-destructive testing (NDT). Moreover, the elements were operated at 70 V DC biasing and driven with a 5 V narrow pulse provided through a USB connection, making the CMUTs suitable for portable devices. The proposed reduced-gap architecture lowers the needed operating voltage for the CMUT elements resonating at high frequencies. This is illustrated here through an analysis of the CMUT operating principle. Finite element simulations show that the proposed reduced-gap design provides a ∼4× bias voltage supply reduction over the traditional architecture to achieve the required vibration, leading to a sufficient acoustic pressure. Acoustic measurements of the proposed CMUT in-air show a 3.33 MHz resonance frequency with a ranging distance up to 27 mm. The CMUT element was sealed using a Parylene-C coating under-vacuum for immersion-applications. In an underwater pulse-echo setup, the backplate-echo of a 3 mm thick aluminum plate was detected. Moreover, the Parylene-C coating served as a method for increasing the fractional bandwidth (BW) by more than 100% at the expense of shifting the CMUT resonance to a higher frequency up to 4.55 MHz. Such immersed operation is promising for non-destructive testing (NDT) applications.