Background: Allergic rhinitis is one of the most common chronic diseases in children. Although it has a large impact on the patient's quality of life, little is known about the factors associated with its severity. The aim of this study was to assess the factors associated with the severity of rhinoconjunctivitis among children in the general population. Methods: A survey was conducted using an online research panel in 2012. Parents were asked to answer an International Study of Asthma and Allergies in Childhood-based questionnaire to identify children with current rhinoconjunctivitis and evaluate factors associated with the severity of its symptoms. Severity was rated according to the degree of impairment caused by the symptoms in the patient's daily life. Results: Among 26,725 children aged 6–12 years old, rhinoconjunctivitis was defined in 5175 (19.4%), and of these, 688 children (13.3% of children with current rhinoconjunctivitis) presented severe symptoms. Living in areas with a high cedar and cypress pollen count and having concurrent eczema were associated with severe rhinoconjunctivitis [adjusted OR (95% CI): 1.21 (1.00–1.46) and 1.45 (1.20–1.75), respectively]. Further, a maternal history of asthma and allergic rhinitis was a significant risk factor for severe rhinoconjunctivitis [1.34 (1.04–1.74) and 1.30 (1.10–1.53), respectively]. However, living with fur-bearing animals (pets) before 1 year of age proved to be a protective factor against severe rhinoconjunctivitis [0.70 (0.52–0.94)]. Conclusions: Environmental factors such as pets and pollen, together with comorbidities and a maternal history of allergic diseases, play an important role in determining the severity of rhinoconjunctivitis.