Objective To compare the diagnostic accuracy of transvaginal ultrasound (TVS) and magnetic resonance imaging (MRI) for assessing myometrial infiltration (MI) in patients with low grade endometrioid endometrial cancer. Methods Observational prospective study performed at a single tertiary care center from 2016 to 2020, comprising 156 consecutive patients diagnosed by endometrial sampling as having an endometrioid grade 1/grade 2 endometrial cancer. TVS and MRI were performed prior to surgical staging for assessing MI, which was estimated using subjective examiner's impression and Karlsson's method for both TVS and MRI. During surgery, intraoperative assessment of MI was also performed. Definitive pathological study considered as reference standard. Diagnostic accuracy for ultrasound, MRI, and intraoperative biopsy was estimated and compared. Results Sensitivity and specificity of TVS for detecting deep MI were 75 and 73.5% for subjective impression and 65 and 70% for Karlsson method, respectively (P = .54). Sensitivity and specificity of MRI for detecting deep MI were 80 and 87% for subjective impression and 70 and 71.3% for Karlsson method. MRI subjective impression showed a significant better specificity than MRI Karlsson method (P = .03). MRI showed better specificity than TVS when subjective impression was considered (P <.05), but not for Karlsson method. Sensitivity and specificity of intraoperative were 75 and 97%, respectively. Intraoperative biopsy showed better specificity than ultrasound and MRI either using examiner's impression or Karlsson method (P <.05). Conclusions MRI revealed a significant higher specificity than TVS when assessing deep myometrial infiltration. However, the intraoperative biopsy offers a significant better diagnostic accuracy than preoperative imaging techniques.