Purpose This study aimed to investigate the effects of posture while using a smartphone on inspiratory capacity and cervical and thoracic posture in university students. Methods Thirty subjects (18 women and 12 men) participated in this study. The participants were asked to sit in a normal sitting position without using a smartphone, followed by sitting while watching a video using a smartphone. The angle was measured using an iPhone application (Goniometer Pro) to quantitatively assess cervical and thoracic flexion range of motion. We measured inspiratory capacity (strength index, peak inspiratory flow, and volume) using the Powerbreathe K5. Results Cervical and thoracic flexion angles were significantly greater when sitting while using a smartphone relative to sitting without using a smartphone (p <0.05). The strength index, peak inspiratory flow, and volume after smartphone use were significantly lower when compared to the period prior to smartphone use (p <0.05). Conclusion Poor posture while using a smartphone may be a risk factor for spinal musculoskeletal and inspiratory disorders.