Citizens of the Republic of Serbia and foreign nationals with approved temporary or permanent residence in the Republic of Serbia arriving from any country in the world with an unfavourable epidemiological situation, who do not hold a negative result of the RT-PCR test for the presence of the SARS-CoV-2 virus, issued by the reference laboratory of the country from which they are coming and/or entering the Republic of Serbia, or persons arriving from the United States of America who do not hold a negative Antigen FIA Rapid test result issued within the previous 48 hours, will be issued a written notice during passport control ― a health warning of their obligation to adhere to the 10-day home-quarantine measure, as of the date of crossing the state border, and will be warned of the obligation to register with the competent COVID-19 infirmary or the territorially competent institute for public health, through this portal: www.e-zdravlje.gov.rs, within 24 hours of crossing the state border.

Source https://www.mfa.gov.rs/en/citizens/travel-serbia/covid-19-entry-requirements