Background: COVID-19 has changed the world and strongly affected the health of the people and the quality of their life. These changes might impact employees’ physical activity (PA) and sedentary behavior (SB). This study aimed to summarize the literature focusing on the COVID-19-caused changes in physical activity and sedentary behavior among the adult working population. Methods: Literature searches were conducted in June 2021 using multiple electronic databases. The following keywords and synonyms were used during the searching process: physical activity, sedentary behavior, COVID-19, employee. After the data cleaning process (duplicates, inclusion criteria), the title and the abstract of all manuscripts from the searches were screened independently by two reviewers. Results: Thirty-nine manuscripts were selected as a result of the searching process. Of these, 5 were SB related, 15 were PA related and 19 addressed both PA and SB. There were longitudinal (10), cross-sectional (28) and a case study (1) in the selected manuscripts. The majority of studies were conducted in the USA (6) and Europe (18), and four studies examined the PA and/or SB in multiple countries. The majority (34 studies) of the studies used subjective, self-reported, but mostly before-validated questionnaires. Objective measures were less common and used only 12.8% of the examined studies. Moreover, 76.4% of the studies described an overall decrease in the amount of PA during the COVID-19 pandemic. In three cases, researchers observed an increase in PA among the workers. Five studies reported no significant changes in the amount of PA during the pandemic. As far as SB is concerned, 18 out of 24 of the studies reported an overall increase in the amount of SB between the two periods. Four manuscripts reported no significant change in the amount of SB, and there was only one manuscript in the examined studies that reported an overall decrease in the SB time before and during the COVID-19 pandemic. Discussion: There is no doubt that the COVID-19 pandemic, including lockdown and work from home (WFH) policies, impaired the PA and SB level of the populations. This period has delivered an important message for the adult working population as well. They should be as active as possible and avoid high levels of SB and uninterrupted sitting time. Therefore, organizations, policies and public health bodies should motivate workers, especially office workers, to be more active and interventions must be developed to mitigate the negative effects of the COVID-19 on PA and SB.