Real-time monitoring of substrate parameters in the root-zone through dielectric sensors is considered a promising and feasible approach for precision irrigation and fertilization management of greenhouse soilless vegetable crops. This research investigates the effects of timer-based (TIMER) compared with dielectric sensor-based irrigation management with different irrigation set-points [SENSOR_0.35, SENSOR_0.30 and SENSOR_0.25, corresponding to substrate volumetric water contents (VWC) of 0.35, 0.30 and 0.25 m3 m−3, respectively] on water use, crop performance, plant growth and physiology, product quality and post-harvest parameters of soilless green bean (Phaseolus vulgaris L., cv Maestrale). In SENSOR treatments, an automatic system managed irrigation in order to maintain substrate moisture constantly close to the specific irrigation set-point. The highest water amount was used in TIMER treatment, with a water saving of roughly 36%, 41% and 47% in SENSOR_0.35, SENSOR_0.30 and SENSOR_0.25, respectively. In TIMER, the leaching rate was ≈31% of the total water consumption, while little leaching (<10%) was observed in SENSOR treatments. TIMER and SENSOR_0.35 resulted in similar plant growth and yield, while irrigation set-points corresponding to lower VWC values (SENSOR_0.30 and SENSOR_0.25) resulted in inadequate water availability conditions and impaired the crop performance. The study confirms that rational sensor-based irrigation allows to save water without compromising anyhow the product quality. In SENSOR irrigation management, in fact, especially in the case of optimal water availability conditions, it was possible to obtain high quality pods, with fully satisfactory characteristics during storage at 7 °C for 15 days.