We report on the realization of ultrasensitive absolute pressure sensors based on silicon nitride membrane sandwiches. These sandwiches consist in a pair of highly-pretensioned, ultrathin (50 nm), large area (0.25 mm2) films, suspended parallel to each other and forming an ultrashort (500 nm) open cavity. The compression of a gas in this cavity leads to a strong squeeze film force, resulting in an increase in the membrane mechanical resonance frequencies, which is directly proportional to the absolute gas pressure. These sandwiches show a record high responsivity of >300 Hz/Pa in terms of squeeze film-induced frequency shift in the range 10−3-100 Pa, which, combined with high quality factor mechanical resonances (Q>106), allows for bringing the sensitivity of absolute squeeze film pressure sensors down to the sub-millipascal level. Display omitted •SiN membrane sandwich-based absolute gas pressure sensor with record responsivity/sensitivity.•Observation of species-independent squeeze film-induced mechanical resonance frequency shifts.•Observation of species-dependent squeeze film-induced mechanical damping.