Starts from 20 December 2020 it is mandatory for foreign citizens to have a negative PCR test not older than 48 hours to enter Republic of Serbia.

Citizens of Serbia, can enter Serbia, however, they will be required to stay in home isolation for 10 days if they do not have a negative PCR test not older than 48 hours. The quarantine can be interrupted once they provide a negative PCR test.

https://www.srbija.gov.rs/vest/en/164948/mandatory-pcr-test-for-foreigners-entering-serbia-as-of-20-december.php