The environmental conditions and the humidity level are crucial factors in caring for artworks. The aim of this work is to propose a method for on-site non-invasive moisture monitoring of wooden artworks or structures. In this regard, a truncated open-ended coaxial probe was designed, implemented, and tested to sense (in combination with a miniaturized Vector Network Analyzer) the variations of water content in woods and stones. More in detail, for the experimental tests, two types of wood (seasoned fir and not seasoned fir) and a limestone, used in Italian Artworks and structures, were analyzed at different moisture levels.