This study evaluated in vivo, an intraoral device against the gold standard esophageal device for gastroesophageal reflux disease (GERD) monitoring. Subjects scheduled for a catheter-based esophageal pH/impedance testing at a gastroenterology clinic were recruited. They were screened using the GerdQ questionnaire, demographics and dental conditions recorded. A prototype intraoral device, consisting of a Bravo™ capsule embedded in an Essix-style retainer fabricated for each subject, monitored intraoral pH. Concurrently, subjects underwent 24-h esophageal pH-impedance monitoring. A self-administered survey elicited the comfort and acceptance of both devices. The study recruited ten adult subjects (23 to 60-years-old) with a median GerdQ score of 9.5 corresponding to a 79% likelihood of GERD. Subjects with severe dental erosion had significantly (p < 0.05) higher acid exposure time and more non-meal reflux events. No adverse events were associated with the intraoral device while one was recorded for the esophageal device. The intraoral device was significantly more comfortable to place, more comfortable to wear, and interfered less with daily routine compared to the esophageal device. Accuracy of the intraoral device ranged between 86.15% and 37.82%. Being more tolerable than traditional esophageal pH monitoring, intraoral pH monitoring may be a useful adjunct for the diagnosis and management of GERD.