Background: The study aimed to develop dengue severity score to assess severe manifestations among hospitalized patients with dengue infection. Method: Children and adolescents with serologically confirmed dengue infection admitted at Ramathibodi Hospital from 2004 to 2018 and treated by an expert multidisciplinary team were recruited. Medical records were retrospectively reviewed and 14 items, related to clinical parameters and managements during hospitalization, were obtained daily as dengue severity score. Results: A total of 191 patients with a mean age of 10.7 years from 2004 to 2013 were recruited. They were classified as dengue fever (35), dengue hemorrhagic fever (DHF) I (53), II (50), III (37) and IV (16). The analysis of 593 daily records revealed the range of daily severity score among patients with DHF grades III (10–20) and IV (31–47) were significantly higher than those of other groups (dengue fever, 5–13; DHF I, 2–10; DHF II, 6–11) with P-values of 0.0001. Using a validity test, a total daily score of ≥12 was an assessment tool for dengue shock syndrome with sensitivity, 86% and specificity, 84%. An additional 51 hospitalized patients with DHF grades II, III and IV with similar ages from 2014 to 2018 were recruited. The number of patients with severe manifestations, having daily score of ≥12, was significantly higher than those without severe manifestations starting from Day −3 to Day +1 of illness. Conclusions: Daily dengue severity score of ≥12 was an accurate assessment tool for severe manifestations.