Update 2021-05-17: According to information from the Federal Foreign Office on Monday (17 May), travellers must present a COVID-19 vaccination certificate, a negative PCR test no more than 72 hours old, or proof of a COVID-19 infection between two and nine months old. Travellers entering the country via the international airports, the seaports of Igoumenitsa, Corfu and Patras or the land border crossings of Evzoni (North Macedonia), Ormeni and Promachonas (Bulgaria) are exempt from the quarantine obligation. Different rules apply at some of the various border crossings, for details see the link below.
Auswärtiges Amt