Early oral cancer is preferably treated by surgery. Its complete removal is essential for locoregional control and disease-free survival. Inadequate resection margins require adjuvant therapy such as re-resection or (chemo)radiation, that causes extra morbidity and oral discomfort. Intraoral ultrasonography (US) is reported to be of value in determining tumor thickness. Intraoperative visualization of the tumor may facilitate the resection and ensure adequate surgical margins. Furthermore, accurate prediction of tumor thickness could help determine the treatment strategy of the clinically node-negative neck, as thickness and depth of invasion are predictors of cervical metastasis as well as prognosticators of survival. The 8th edition of the American Joint Committee on Cancer staging system for oral squamous cell carcinoma has included depth of invasion as parameter for cT-stage. The aim of this review is to analyze the accuracy of intraoral US in determining tumor thickness in oral cancer. A systematic search was conducted, and the quality of the included papers was assessed using the QUADAS-2 tool for diagnostic accuracy studies. Subsequently, a meta-analysis was performed on the available individual participant data of 240 patients. Most of the twelve included studies focused on T1-2 tongue cancer (n = 129). Meta-analysis showed a high correlation in tumor thickness within this subgroup as measured by intraoral US and histopathology (r = 0.82, p <.001), with minor overestimation of 0.5 mm on US. It is concluded that intraoral US is very accurate in determining tumor thickness in early oral tongue cancer.