Background To explore prognostic value of the pre-treatment primary lesion apparent diffusion coefficient (ADC) in locoregionally advanced nasopharyngeal carcinoma (LA-NPC). Methods A total of 843 patients with newly diagnosed LA-NPC were enrolled from January 2011 to April 2014 and divided into two groups based on ADC values: the low-ADC group and high-ADC group. The 3-year local relapse-free survival (LRFS), distant metastasis free survival (DMFS), disease-free survival (DFS) and overall survival (OS) rates between two groups were compared using Kaplan-Meier curve, and Cox regression analyses were performed to test prognostic value of the pretreatment ADC in LA-NPC. Results The cut-off value of the pretreatment ADC for predicting local relapse was 784.5 × 10 − 6 mm 2 /s (AUC [area under curve] = 0.604; sensitivity = 0.640; specificity = 0.574), thus patients were divided into low-ADC (< 784.5 × 10 − 6 ; n = 473) group and high-ADC (≥784.5 × 10 − 6 ; n = 370) group. The low-ADC group had significantly higher 3-year LRFS rate and DFS rate than the high-ADC group (LRFS: 96.2% vs. 91.4%, P = 0.003; DFS: 81.4% vs. 73.0%, P = 0.0056). Multivariate analysis showed that the pretreatment ADC is an independent prognostic factor for LRFS (HR, 2.04; 95% CI, 1.13–3.66; P = 0.017) and DFS (HR, 1.41; 95% CI, 1.04–1.89; P = 0.024). Conclusions The pretreatment ADC of the primary lesion is an independent prognostic factor for LRFS and DFS in LA-NPC patients. Electronic supplementary material The online version of this article (10.1186/s12885-019-5684-3) contains supplementary material, which is available to authorized users.