Many studies observed a reduction of physical activity (PA) and an increase in digital media use in young adults during the COVID-19 pandemic. However, few studies have been conducted in Europe or looked at changes in the association between both behaviors. Hence, this study aims at investigating the changes in digital media use/social media use and PA as well as in its association among young adults in Germany. Cross-sectional data of 884 German young adults (mean age 22.36 (±1.99), 76% female) collected via an online questionnaire between August 1 and September 30, 2020 were analyzed. Participants reported on digital media use (smartphone, television, computer, gaming console), social media use (Facebook, Instagram, Snapchat, Twitter, YouTube, TikTok) and PA (days/week of ≥30 min. PA) separately for the period of strict infection control measures in Germany (March - end of May 2020) and for normal times (before March 2020). Descriptive statistics of digital media use, social media use and PA were compared between both periods. Linear regression adjusted for sociodemographic and work-related characteristics were conducted for both periods with total media use, the various media devices and social media use, respectively, as independent and PA as dependent variables. Whereas PA did not differ between both periods, mean total digital media use increased by 1 hour during the period of strict infection control measures. Digital media use and social media use were negatively associated with PA in both time periods. Differences in these associations by sex could be found for some digital media devices. However, 60% of respondents did not comply with the WHO recommendations for PA. Under consideration of possible recall bias, young adults’ digital media use, but not PA, seemed to have changed under the strict infection control measures. However, interventions are needed to increase PA and to prevent its reduction in the course of the pandemic.