Background: Portable, small computers and smartphones are now considered essential tools in modern society and smartphone ownership and usage rates are rising every year. However, excessive smartphone use can have musculoskeletal and postural implications, leading to “smartphone addiction” and related dysfunctions. Objects: This study aimed to investigate the effects of smartphone addiction on scapular position and muscle activity during shoulder abduction in asymptomatic individuals. Methods: A total of 45 participants were classified into high-risk, middle-risk, and low-risk groups based on their smartphone addiction levels. Scapular position was measured using the scapular index, round shoulder posture (RSP), lateral scapular slide test, and scapulohumeral rhythm spine angle. Muscle activity was assessed using electromyography of the upper trapezius (UT), lower trapezius (LT), serratus anterior (SA), and anterior deltoid (AD) muscles during shoulder abduction. Results: Smartphone addiction was significantly associated with altered scapular position and muscle activity. The high-risk group exhibited greater forward head posture and more pronounced RSP. Additionally, the high-risk group had lower SA activation and higher UT, LT, and AD muscle activity, indicating compensatory mechanisms due to altered scapular positioning. Conclusion: These findings suggest that excessive smartphone use contributes to postural deviations and altered muscle activation patterns, which may lead to musculoskeletal dysfunction over time. Clinicians should consider smartphone use when assessing patients with scapular dysfunction, and future studies should explore interventions to mitigate these effects.