There have been concerns that some patients with sepsis—life-threatening organ dysfunction caused by a dysregulated host response to infection—may be overlooked with a quick Sequential Organ Failure Assessment (qSOFA)≥2 in the emergency department (ED). Despite this, little is known about the risk factors associated with sepsis among patients with qSOFA<2 in the ED. This is a retrospective cohort study using ED data from a large tertiary medical center in Japan 2018- 2020. We included adult patients (aged ≥18 years) presenting to the ED with suspected infection (eg, having a fever) and qSOFA<2. The primary outcome was the diagnosis of sepsis based on the Sepsis-3 criteria (defined as septic patients). We compared patient characteristics (eg, demographics, vital signs upon the initial triage, chief complaint, and comorbidities) between septic and non-septic patients. Additionally, we identified the potential risk factors of sepsis among patients with qSOFA<2 using a multivariable logistic regression model. We identified 151 (7%) septic patients among 2, 025 adult patients with suspected infection and qSOFA<2. Compared with non-septic patients, septic patients were likely to be older and have vital signs suggestive of imminent sepsis (eg, high respiratory rate). In the multivariable logistic regression model, the potential risk factors of sepsis among patients with qSOFA<2 were older age (adjusted OR, 1.92 [95% CI 1.19- 3.19]), vital signs suggestive of imminent sepsis (eg, adjusted OR of altered mental status, 3.50 [95% CI 2.25- 5.50]), receipt of oxygen therapy upon arrival at the ED (adjusted OR, 1.91 [95% CI 1.38-2.26]), chief complaint of sore throat (adjusted OR, 2.15 [95% CI 1.08-4.13]), and the presence of comorbid diabetes mellitus, ischemic heart disease, and chronic kidney disease (eg, adjusted OR of diabetes mellitus, 1.47 [95% CI 1.10-1.96]). On the contrary, high systolic blood pressure, and chief complaint of abdominal and chest pain were associated with a lower risk of sepsis (eg, adjusted OR of abdominal pain, 0.26 [95% CI 0.14-0.45]). We found that older age, vital signs prognosticating sepsis, and the presence of some comorbidities were the potential risk factors of sepsis in patients with qSOFA<2. To prevent missed diagnoses of sepsis, we should treat patients with these potential risk factors more cautiously.