A highly sensitive ultrasound sensor based on an integrated photonics Mach-Zehnder interferometer (MZI) fabricated in silicon-on-insulator technology is reported. The sensing spiral is located on a membrane of size 121 μm×121 μm. Ultrasound waves excite the membrane's vibrational mode, which translates to modulation of the MZI transmission. The measured sensor transfer function is centered at 0.47 MHz and has a -6 dB bandwidth of 21.2%. The sensor sensitivity is linear in the optical input power and reaches a maximum 0.62 mV/Pa, which is limited by the interrogation method. At 0.47 MHz and for an optical power of 1.0 mW the detection limit is 0.38 mPa/Hz1/2 and the dynamic range is 59 dB. The MZI's gradual transmission function allows a wide range of wavelength operation points. This strongly facilitates sensor use and is promising for applications.