Background. During early 2020, because of the COVID-19 pandemic and related lockdown, most education systems-including universities-shifted from face-to-face classes to online learning. In Saudi Arabia, this might have contributed to a decreased level of physical activity (PA) and a concurrent increase in sedentary behaviour among young adults. This study aimed to investigate the impact of online learning on PA during the COVID-19 lockdown period among female undergraduate students in Saudi Arabia. Methods. Data were collected through an online survey administered to partici-pants. It consisted of three sections including demographic information, participants' perception towards online learning and PA, and PA level using the self-reported active-questionnaire survey tool. The association between online learning and PA was measured using linear regression. The statistical significance was set at P < 0.05. Results. A total of 197 female undergraduate students were included; 95.4% of them were aged 18-24 years old, and 59.9% were in the normal body mass index range (18.5- 24.9 kg/m(2)). In terms of PA level, 55.3% were highly active, 33.5% were moderately active and 10.1% were low-active. In terms of students' perception of engaging in PA, 53.3% of students reported that engaging in PA definitely affected their psychological status. Moreover, compared with those attending <25 hours/week of online learning, those who attended >30 hours/week had lower PA (r = -363.24; 95% confidence interval (CI) -593.97, -132.50), followed by those attending 25-30 hours/week (r = -277.66; 95% CI -484.65, -70.66). Conclusion. Online learning has negatively affected the PA level of female under-graduate students in Saudi Arabia during the COVID-19 lockdown period, in a dose -dependent manner. Moreover, this might affect their phycological status. Nevertheless, future studies are warranted to further investigate the relationship between PA level and psychological status.