OBJECTIVE: The aim of this study was to investigate the relationship between smartphone usage and neck extensor muscle thickness through ultrasonographic evaluation. METHODS: A prospective cross-sectional study was conducted with 75 participants aged 18–40 years. Demographic data, smartphone usage patterns, and neck extensor muscle thickness were assessed. Smartphone addiction was evaluated using the short version of the Smartphone Addiction Scale. Statistical analyses included appropriate tests, with p<0.005 being considered statistically significant. RESULTS: Participants with smartphone addiction demonstrated statistically significantly higher Neck Disability Index scores compared to their non-addicted counterparts (p=0.001). Although no statistically significant differences were observed in muscle thickness between the addicted and non-addicted groups, smartphone usage duration negatively correlated with dominant trapezius thickness (ρ=-0.362, p=0.028). The age of smartphone use onset was positively correlated with the thickness of several muscles, including the splenius capitis on both the dominant (ρ=0.358, p=0.030) and non-dominant sides (ρ=0.487, p=0.002), and the average thickness of the splenius capitis (ρ=0.442, p=0.006) and semispinalis cervicis muscles (r=0.349, p=0.034). Regression analysis confirmed significant relationships between the age of onset and neck extensor muscle thickness. CONCLUSIONS: Prolonged smartphone use, especially when initiated at an early age, is associated with changes in neck extensor muscle morphology. Ultrasonographic evaluation revealed correlations between usage patterns and muscle thickness, highlighting the importance of preventive measures and ergonomic interventions to mitigate potential musculoskeletal risks associated with excessive smartphone use.