In Thailand and worldwide, smartphone addiction among university students is a growing concern. This study aims to investigate behaviors of smartphone use, the prevalence of smartphone addiction, the duration of smartphone use, and their associated factors among pharmacy students at a university in northern Thailand. This cross-sectional study was conducted using an online self-administered questionnaire to collect data from January to February 2021. Smartphone addiction was measured using the Smartphone Addiction Scale: Thai Short Version (SAS-SV-TH). Of 281 students (70% female, average age of 21.1 (2.0), year 1 to 5), 87% used smartphones and tablets. Their average time spent on a smartphone was 7.5 (±3.1) hours daily on weekdays and 8.1 (±3.1) on weekends. The top three reasons for using smartphones were social networking (92.9%), education (90.3%) and entertainment (89.6%). Health-related problems associated with smartphone use were insomnia (51.3%), anxiety (41.3%), headache (38.8%) and stress (38.4%). The prevalence of smartphone addiction was 49% (95% CI: 44–55%); the associated factor comprised time spent on smartphones (>5 h/day). The prevalence of spending more than five hours daily on smartphones was 75% (95% CI: 70–80%) during weekdays and 81% (95% CI: 77–86%) during weekends; associated factors for during weekdays included a monthly smartphone bill of more than 500 THB (adjusted odds ratio: 4.30 (95% CI: 2.00–9.24) and for senior students (adjusted OR: 3.31 (95% CI: 1.77–6.19). The results remained the same for the weekend. In short, the results show that half of the pharmacy students were addicted to their smartphone; time spent on smartphones (>5 h/day) was associated with addiction. Therefore, university students should be encouraged to adopt healthy habits for smartphone use (such as limiting screen time and maintaining good posture while using a smartphone or tablet) and to increase their awareness of health-related problems.