Introduction: Coronavirus disease (COVID-19) is an infectious disease caused by the SARS-CoV-2 virus. This disease harmed people's lives, compromising a healthy lifestyle. Objective: This study aims to analyze the impact of COVID-19 lockdown on the Portuguese population's physical activity and seated time during quarantine. Methodology: This retrospective longitudinal study was based on an online Google Forms survey concerning the lockdown period from 18th March to 30th April, 2020. A total of 541 responses were analyzed; all Portuguese (42.2 ± 14.8 years) completed the survey concerning the period. The valid responses were obtained from the 4th to 30th of May 2020. The physical activity during a week and the average seated time on a weekday before (T0) and during (T1) the quarantine were studied. Results: When comparing T0 and T1, there was a decrease in moderate-vigorous physical activity (p < 0.0001), walking time (p < 0.0001), and an increase in seated time (p < 0.0001), considering the whole sample group. All age groups decreased the time spent walking and increased the time seated. The highest value of seated time was displayed in the age interval of 26–35. Compared to the others, the age interval of 46—55 years showed the lowest values for vigorous and moderate activity. Conclusion: The first quarantine had a significantly negative effect on the Portuguese population's physical activity hours and contributed to a significant increase in the seating time during a weekday.