House dust mite (HDM) allergens are responsible for the most prevalent persistent respiratory allergies. Clinical trials in this field often use a four-component nasal symptom score (T4NSS) as a measure of efficacy. The present observational, prospective, multinational, multicenter study determined the minimal important difference (MID) for a T4NSS in children, adolescents, and adults with physician-diagnosed HDM-induced allergic rhinoconjunctivitis (AR). Patients rated the T4NSS daily, a 15-point global rating of change scale (GRCS) and the Rhinoconjunctivitis Quality of Life Questionnaire weekly. The MID was determined primarily by using a regression method with a GRCS threshold of 2. A total of 546 patients (210 adults, 133 adolescents, and 203 children) were included; 92.6% of the patients had moderate-to-severe AR, and 30.1% had concomitant mild asthma. During the first week, the mean ± standard deviation T4NSS was 5.68 ± 2.76 in adults, 5.34 ± 2.66 in adolescents, and 5.07 ± 2.48 in children. In a GRCS regression analysis, the MID [95% confidence interval] for the T4NSS was -0.90 [-1.06;-0.75] overall (n = 509), -0.94 [-1.19;-0.69] in children (n = 187), -0.74 [-1.07;-0.41] in adolescents (n = 125), and -1.04 [-1.29;-0.79] in adults (n = 197). The MID did not differ greatly from one disease severity tertile to another. Confirmatory distribution-based analyses yielded overall MID values of -0.87 for the first week of the study and -0.93 for the week 2-week 1 difference. The MID for improvement in the T4NSS is at least -0.90 units in children, adolescents, and adults suffering from HDM-induced AR This value could be rounded up to -1 unit for convenience.