This cross-sectional study identified the association between COVID-19-related perceived household financial decline and smartphone dependency among adolescents in South Korea. Data from the 2020 Youth Risk Behavior Survey of Korea was used and 54,809 middle and high school students were included. COVID-19-related perceived household financial decline was categorized as no financial decline, mild, moderate, and severe. Smartphone dependency was calculated by 10 questions and was largely categorized as yes and no, and as normal, low, and high (prevalence rate: 25.0%). Binary and multinomial regression analyses were performed to analyze the association. The more severe the financial decline, the more pronounced the risk of high-risk smartphone dependency (mild financial decline: odds ratio (OR) 1.11, 95% CI 0.96–1.28; moderate: OR 1.22, 95% CI 1.04–1.43; severe: OR 2.56, 95% CI 2.06–3.17). Poor family relationships (OR 1.06, 95% CI 1.03–1.10) and severe social conflict (OR 2.99, 95% CI 2.50–3.58) were also related to smartphone dependency. The ORs were 2.63 with more than three bathrooms and 1.63 with their own bedroom. Smartphone dependency among adolescents is closely related to COVID-19-related perceived household financial decline. As smartphone dependency relates to complicated psychological issues, further evaluation is necessary, especially for vulnerable adolescents.