Background: The Public Health Services in the Rotterdam region, the Netherlands, observed a substantial decrease of non-COVID-19 notifiable infectious diseases and institutional outbreaks during the first wave of the COVID-19 epidemic. We describe this change from mid-March to mid-October 2020 by comparing with the pre-COVID-19 situation. Methods: All cases of notifiable diseases and institutional outbreaks reported to the Public Health Services Rotterdam-Rijnmond between 1st January and mid-October 2020 were included. Seven-day moving averages and cumulative cases were plotted against time and compared to those of 2017–2019. Additionally, Google mobility transit data of the region were plotted, as proxy for social distancing. Results: Respiratory, gastrointestinal, and travel-related notifiable diseases were reported 65% less often during the first wave of the COVID-19 epidemic than in the same weeks in 2017–2019. Reports of institutional outbreaks were also lower after the initially imposed social distancing measures; however, the numbers rebounded when measures were partially lifted. Conclusions: Interpersonal distancing and hygiene measures imposed nationally against COVID-19 were in place between mid-March and mid-October, which most likely reduced transmission of other infectious diseases, and may thus have resulted in lower notifications of infectious diseases and outbreaks. This phenomenon opens future study options considering the effect of local outbreak control measures on a wide range of non-COVID-19 diseases. Targeted, tailored, appropriate and acceptable hygiene and distancing measures, specifically for vulnerable groups and institutions, should be devised and their effect investigated.