[Purpose] This study aimed to examine the influence of social-networking service usage via smartphone on internet addiction and psychological stress in Japanese university students studying physical therapy. [Participants and Methods] This single-university cross-sectional study involved 247 physical therapy students in the second to fourth years (ages 19 to 22). By use of self-administered questionnaires, we collected information on daily time of smartphone usage, social-networking service usage via smartphone, and daily self-learning time outside of class hours. We assessed internet addiction and psychological stress using the Internet Addiction Test and Stress Response Scale-18, respectively. After excluding twelve participants, we analyzed the data collected for the other 235. [Results] Multiple regression analysis showed an association of the Internet Addiction Test score with gender and daily time of smartphone usage. "Surfing without any purpose", which is one of the purposes of social-networking service usage, and the Internet Addiction Test score were associated with the Stress Response Scale-18 score. Other variables were not associated with the Internet Addiction Test or Stress Response Scale-18 scores. [Conclusion] Our results suggest that gender (males), longer time of smartphones usage, or using social-networking service usage passively cause internet addiction or psychological stress in Japanese physical therapy university students.