•Adjuvant radiotherapy improved locoregional control and survival for node-negative high grade parotid gland cancer.•Radiotherapy is recommended for low grade parotid gland cancer with advanced T stage or involved resection margins.•Histologic grade is an independent prognostic factor for node-negative parotid gland cancer.•Higher biologic effective dose (≥77 Gy10) is associated with increased disease control. To determine the role of adjuvant radiotherapy (ART) in parotid gland cancer without nodal metastasis, we evaluated the survival outcomes, prognostic factors, and dose–response relationships in patients with node-negative parotid gland cancer patients. Patients who underwent curative parotidectomy and were pathologically diagnosed with parotid gland cancer without regional or distant metastases between 2004 and 2019 were reviewed. The benefit of ART in terms of locoregional control (LRC) and progression-free survival (PFS) were evaluated. In total, 261 patients were included in the analysis. Of them, 45.2 % received ART. The median follow-up period was 66.8 months. Multivariate analysis revealed that histological grade and ART were independent prognostic factors for LRC and PFS (all p <.05). For patients with high-grade histology, ART was associated with a significant improvement in 5-year LRC (p =.005) and PFS (p =.009). Among patients with high-grade histology who completed RT, higher biologic effective dose (≥77 Gy10) significantly increased PFS (adjusted hazard ratio HR, 0.10 per 1-Gy increase; 95 % confidence interval CI, 0.02–0.58; p =.010). ART significantly improved LRC (p =.039) in patients with low-to-intermediate histological grade as well per multivariate analysis, and subgroup analyses revealed patients with T3–4 stage and close/positive resection margins (<1 mm) would benefit from ART. ART should be strongly recommended for patients with node-negative parotid gland cancer with high-grade histology in terms of disease control and survival. In patients with low-to-intermediate-grade disease, those with high T stage and incomplete resection margin benefit with ART.