Micro scanning mirrors that can operate reliably under water is useful in both ultrasound and photoacoustic microscopic imaging, where fast scanning of focused high-frequency ultrasound beams is desired for pixel-by-pixel data acquisition. This paper reports the development of a new micro-fabricated water-immersible scanning mirror with a small form factor. It consists of an optically and acoustically reflective mirror plate, which is supported onto two flexible polymer hinges and driven by an integrated electromagnetic micro-actuator. It can achieve one-axis scanning of ±12.1° at a resonant frequency of 250Hz in air and 210Hz in water, respectively. By optimizing the design and enhancing the fabrication with high-precision optical 3D printing, the overall size of the scanning mirror module is less than 7 mm × 5 mm × 7 mm. The small form factor, large scanning angle, and high resonant frequency of the new water-immersible scanning mirror make it suitable for building compact handheld imaging probes for in-vivo high-speed and wide-field ultrasound and photoacoustic microscopy.