Introduction: Increased use of smartphones can lead to smartphone addiction and “no-mobile-phone-phobia” known as nomophobia. During an ongoing coronavirus disease 2019 (COVID-19) lockdown, to tackle an issue of boredom, many people including undergraduate health sciences students (HSSs) are using smartphones, leading to an increased risk of smartphone addiction and nomophobia. During lockdown, many universities have shifted the mode of learning to e-classes by using an internet-based technology on smartphones, which may contribute to increased exposure to smartphones, leading to an increased risk of smartphone addiction among students. Objective: To assess an impact of COVID-19 lockdown on smartphone addiction and severity of nomophobia among UG HSSs. Materials and Methods: The present study was conducted at a rural tertiary health-care center in Maharashtra state of India with a sample size of 412. Smartphone Addiction Scale–Short Version was used to assess smartphone addiction and Nomophobia Questionnaire was used to assess nomophobia. Results: Majority (69.7%) reported inability to concentrate on studies at home due to increased use of smartphone during the COVID-19 lockdown. Boredom (32.8%) was the most bothering thing during lockdown, and to tackle it, 55.8% reported of using smartphone. 97.6% had nomophobia and 45.1% had smartphone addiction. There was increase in the prevalence of smartphone addiction and nomophobia during the COVID-19 lockdown as compared to the prevalence from studies which were conducted in the pre-COVID-19 lockdown period. Conclusion: Appropriate measures should be taken by health sciences universities and authorities to address an issue of smartphone addiction and nomophobia so that students can concentrate on their studies at home during the COVID-19 lockdown.