The campaign of vaccination against COVID-19 in the EU, launched at the turn of 2020 and 2021, is almost from the beginning accompanied by a discussion on the document confirming the fact of receiving the COVID-19 vaccine or having this disease and, consequently, obtaining immunity to the SARS-CoV-2 coronavirus infection. The proposal to introduce such a document, called an EU Digital COVID Certificate, was presented by the European Commission in March 2021. From the point of view of the EU internal market law, the substantive legal requirement to get vaccinated against COVID-19 for the purpose of cross-border use of various types of services, the fulfillment of which has been confirmed by an EU Digital COVID Certificate, is important. The aim of the paper is to examine whether the requirement to get vaccinated against COVID-19 by a citizen of an EU Member State wishing to use services offered in another EU Member State should be regarded as, in principle prohibited, restriction of the freedom to provide services in the EU. The conclusion that this is the case makes it possible to indicate the conditions which must be fulfilled for the requirement in question to be compatible with EU law.