Aim To investigate reliability of the Easy IOTN app between clinicians with different levels of experience in determining Index of Orthodontic Treatment Need (IOTN) Dental Health Component (DHC) and Aesthetic Component (AC) scores from study models. The accuracy of each clinician in discriminating treatment need using the app against the 'gold standard' conventional assessment at the threshold of treatment acceptance criteria was also explored. Materials and methods In total, 150 sets of pre-treatment study models were assessed by six clinicians using the app on two separate occasions (T1 and T2). A single IOTN-calibrated clinician also scored the models using the conventional technique. Clinician scores for both intra- and inter-rater reliability were assessed using Cohen's Kappa. The performance of each clinician in discriminating treatment need using the app against the conventional assessment method at the threshold of treatment acceptance criteria was also assessed using the area under the curve-receiver operating characteristic. Results The intra-rater agreement for the clinician undertaking the conventional assessment of the models was 1.0. Intra-rater agreement scores for clinicians using the Easy IOTN app ranged between 0.37-0.87 (DHC) and 0.22-0.44 (AC). Inter-rater agreement scores at T2 were 0.59 (DHC) and 0.23 (AC). Based on the IOTN DHC, all clinicians displayed an excellent level of accuracy in determining malocclusions qualifying for treatment (range 81.7-90.0%). Based on the IOTN AC, all clinicians showed an acceptable level of accuracy in determining malocclusions qualifying for treatment (range 71.9-79.2%). Conclusions The Easy IOTN app was shown to have moderate inter-rater reliability. Variation in the intra-rater reliability was evident between clinicians of different grades/level of experience. Importantly, the diagnostic accuracy of the app to discriminate between malocclusions that qualify for NHS treatment was rated as excellent (IOTN DHC) and acceptable (IOTN AC) and independent of clinician grade or level of experience.