Purpose: This study aimed to compare the diagnostic performance of cone-beam computed tomography (CBCT) and digital periapical radiography in detecting separated endodontic instruments (SI) according to their type and length infilled root canals of mandibular molars. Materials and Methods: Forty-two extracted mandibular molars were divided into 2 groups: control (without SI) and experimental (with SI). In the experimental group, SI fragments (2 or 3 mm) from 3 endodontic instruments (WaveOne Gold, Reciproc Blue, and ProTaper Next) were inserted into the mesiobuccal canals of 30 teeth. CBCT scans were obtained using the KAVO OP 3D PRO device, and digital periapical radiographs (orthoradial, distal, and mesial) were acquired using the Focus X-ray unit with the RVG 5200 solid sensor. Three experts independently evaluated all images for SI detection using a 5-point scale. Diagnostic performance metrics were calculated for each imaging method, andcomparisons were performed using 1-way analysis of variance with Tukey post-hoc tests. The significance level wasset at 5% (P = 0.05). Results: Digital periapical radiography demonstrated higher area under the receiver operating characteristic curve (Az), sensitivity, and specificity values than CBCT. ProTaper Next SI on CBCT scans showed the lowest Az values (0.58and 0.56 for 2- and 3-mm fragments, respectively). These values were significantly different from those of the other SIconditions (both CBCT and digital periapical radiography) (P<0.05). Conclusion Digital periapical radiography outperformed CBCT in detecting SI in filled root canals of mandibularmolars and should be the preferred modality. Furthermore, SI detectability varies with imaging modality, depending on instrument type and fragment length.