Unconstrained and continuous oral hygiene monitoring is effective not only in diagnosing oral health status but also in diagnosing systemic health status. A mouthguard-type wearable sensor was proposed to measure salivary turbidity as an indicator of oral hygiene, however, the feasibility of intraoral salivary turbidity measurement was not well investigated. In this study, a mouthguard-type sensor with embedding LED, phototransistor, and wireless module was developed for optical measurement of salivary turbidity. The shape of the turbidity-sensing part of the sensor was designed based on the results of experiments on water-retention capability and measurement sensitivity. In vivo experiments using diluted milk as an ingestible turbidity standard solution showed that the sensor has sufficient quantitative capability (100–1000 FTU) and repeatability in the standard range of salivary turbidity. In addition, by continuously measuring the fluctuation in salivary turbidity using the sensor, clinically useful intraoral information such as salivary secretion and oral dryness status could be obtained. These results demonstrate that the developed mouthguard-type sensor is effective for the unconstrained and continuous evaluation of oral hygiene.