Aim: This descriptive study investigated smartphone addiction and daytime sleepiness in adolescents during the COVID-19 pandemic. Methods: The sample consisted of 1001 adolescents living in the Central Anatolia Region of Türkiye. Data were collected using a sociodemographic characteristics questionnaire, the Cleveland Adolescent Sleepiness Questionnaire (CASQ), and the Smartphone Addiction Scale-Short Version (for Adolescents) (SAS-SV). The data were analyzed using descriptive, comparative, and correlational statistics. Results: Most participants stated that they had spent much more time on their smartphones since the pandemic (87.2%). More than half of the participants noted that they had been on their smartphones for more than 90 minutes daily (63.1%). Less than half of the participants said they spent most of their time on Instagram (44.8%). More than half of the participants reported daytime sleepiness (55.7%). Less than half of the participants had a moderate-level sleep status (44.8%). There was a positive correlation between CASQ “sleep in school” and “sleep in the evening” subscale scores. Conclusion: During the COVID-19 pandemic, adolescents were at high risk of smartphone addiction, affecting daytime sleepiness. Therefore, we need interventions to reduce the risk of psychological problems and daytime sleepiness in adolescents.