Background: Nasopharyngeal carcinoma (NPC) patients receiving concurrent chemoradiotherapy (CCRT) frequently develop low skeletal muscle mass (SMM), but, little is known about the impacts of low SMM on health-related quality of life (QOL). Methods: We retrospectively assessed 56 patients with locoregionally advanced NPC enrolled in a prospective trial. Low SMM was determined on routine computed tomography simulation (CT-sim) scans taken before radiotherapy, at the third cervical (C3) vertebral level with validated sex-specific cutoffs. QOL was assessed using the World Health Organization Quality of Life Questionnaire-100 at baseline and after 3 weeks. Pain was scored every 24 h using a numerical rating scale (NRS). Characteristics related to low SMM were identified by logistic regression. The chi-square test was used to examine the association of low SMM with QOL and pain. Results: Of the 56 participants (mean age 44.20 +/- 10.93 years), over half (60.71%) developed low SMM. Patients with low SMM were more likely to be older (P = 0.035), male (P = 0.066), have a lower body-mass index (BMI; P = 0.091), and have a higher pain score (P = 0.001). Older age (hazard ratio [HR] = 1.788, P = 0.016), being male (HR = 3.145, P = 0.010), lower BMI (HR = 0.761, P = 0.033), and lower prognostic nutritional index (HR = 0.186, P = 0.034) were associated with higher risk of low SMM. Low SMM was associated with poorer baseline QOL scores (P = 0.072), especially in the physical domain (P = 0.002) and its three facets: pain (P = 0.003), energy (P = 0.021), and sleep (P = 0.007). Low SMM was also associated with significantly worse QOL scores (P = 0.006) at 3 weeks, especially in the physical (P = 0.002), psychological (P = 0.046), independence (P = 0.003), social domains (P = 0.023), and in general health condition (P = 0.043). For pain score, low SMM group had worse overall changes from baseline to week 3 (P = 0.011). Conclusions: The incidence of low SMM, as evaluated using routine CT-sim scans, is high in patients receiving CCRT for locoregionally advanced NPC. Low SMM results in poorer QOL and higher pain scores, which underscores the requirement for nutritional and functional interventions to address low SMM early in the treatment course.