Water acts as the solvent for natural biotic and abiotic processes and in many technological contexts. The availability of water to participate in chemical and physical processes is captured by thermodynamic variables which track the energetic state of water such as water activity and water potential. Our understanding of the energetic state of water in relevant processes is limited by a lack of sensors capable of providing accurate and reliable ex situ and in situ measurements of water activity. To address this technology gap, we present applications of a microtensiometer (μTM): a biomimetic microelectromechanical system (MEMS) sensor capable of measuring water activity in liquid, vapor, and semisolid (e.g., hydrogels, cheese) phases. We developed packaging, measurement systems, and methodology to enable us to make water activity measurements previously inaccessible to tensiometry. We present measurements in two contexts: (1) a small benchtop unit for ex situ measurements and (2) a probe format for in situ measurements. We demonstrate that the μTM can accurately measure water activity in a diversity of complex samples and agrees with chilled mirror hygrometry, an industry standard for water activity measurement.