The Greek declaration, dated at 550 or 551 AD, commemorates the founding by the Orthodox abbot Constantine of a building thought to be a hostel for pilgrims, near the city's Damascus Gate.
David Gellman, director of the excavation in east Jerusalem's walled Old City, said that while the area was rich in archaeological finds, few such inscriptions had been found.
The Greek inscription, dated at 550 or 551 AD, commemorates the founding of a building thought to be a hostel for pilgrims near the city's Damascus Gate.
Constantine, the Orthodox priest who founded it, was abbot of the Nea Church, dedicated to the Virgin Mary and the largest church in Jerusalem when it was built in 543 AD.
Gellman said the rectangular section of mosaic, 1.14 metres (3 foot 7 inches) on its longest side, "tells us about the way churches and monasteries worked back then. It makes this piece very unique".
It was discovered intact about a metre (three feet) below street level as Gellman and his team made a routine exploratory dig ahead of the arrival of workers to lay communications cables.
The Old City lies in east Jerusalem, seized by Israel in the 1967 Six-Day War and later annexed in a move never recognised by the international community.
