Precisely and continuously monitoring the moisture content under pavements can provide significant information for pavement condition evaluation and maintenance. In this paper, a moisture sensor using a microwave parallel-plate transmission line is developed and prototyped. The moisture content is determined from the phase shift of waves propagating along the line. The sensor system was installed at an experimental field site, and the moisture contents in the pavement were monitored over a period of six months. Laboratory tests and field tests show that the moisture sensor system is sensitive and accurate. Compared with existing moisture sensors, the developed sensors are very small in size, easy to install, and durable.