Importance Only a few studies have examined the long-term trends of hand and oral hygiene, especially among adolescents.Objective To investigate the 15-year trends in frequency of handwashing and toothbrushing and examine the factors associated with hand and oral hygiene, particularly during the COVID-19 pandemic.Design, Setting, and Participants This cross-sectional study, performed from January 1, 2008, to December 31, 2022, used general population-based data from 963 644 individuals in a national representative survey (Korea Youth Risk Behavior Web-based Survey [KYRBS]).Exposure COVID-19 pandemic.Main Outcomes and Measures Trends in hand and oral hygiene practices were measured by how frequently adolescents washed their hands and whether they fulfilled the recommended guidelines for toothbrushing. An interrupted time series analysis using linear and logistic regression models was performed to assess any associations with the COVID-19 pandemic. Hand and oral hygiene behaviors before and during the pandemic in each sociodemographic subgroup were also compared.Results In the 963 644 adolescents (495 697 [51.4%] male; mean [range] age, 15.01 [12-18] years) who participated in the KYRBS from 2008 to 2022, a 73.3% (95% CI, 59.4%-97.4%; P < .001) immediate increase was seen in overall hand hygiene behavior at the onset of the COVID-19 pandemic compared with the prepandemic period, with a sustained decrease thereafter (beta = -0.018; 95% CI, -0.022 to -0.015; P < .001). Meanwhile, no immediate increase was observed in terms of overall oral hygiene behavior (0.1%; 95% CI, -0.9% to 1.1%; P = .82); however, there was a sustained decrease during the pandemic (beta = -0.018; 95% CI, -0.020 to -0.016; P < .001). Older age, female sex, nonsmoking status, alcohol use, low household economic level, and poor school performance were significantly associated with poor hand hygiene during the pandemic.Conclusions and Relevance In this cross-sectional study of South Korean adolescents, an increase in the prevalence of hand hygiene was observed during the early pandemic; however, this prevalence decreased over time. Meanwhile, the decrease in the prevalence of oral hygiene was more pronounced during the pandemic. This study recommends stronger guidelines for adolescent health coaches, such as teachers or public health advisers, regarding hygiene behaviors, even after the COVID-19 pandemic ends.