Water is a fundamental requirement for life. Its biological availability is partly determined by water activity and thus measuring this parameter is vital in industries such as food, agriculture and beyond. Apart from its terrestrial applications, water activity is also important in planetary exploration, for example in determining habitability in the subsurface of Mars, icy moons and other regions of interest in the Solar System. This paper reports a miniaturized water activity sensor for measurement in planetary environments. In line with space instrumentation constraints, objectives included minimizing the weight and dimensions of the sensor system, lowering the power consumption, enabling smaller sample volumes and achieving faster measurement time. We describe the design, construction, testing and field validation of this instrument.