A circular area near the middle is missing, but authorities say enough fragments have been found to reconstruct a large part.
Correspondents say the unearthing of the tomb at Amphipolis has enthused Greeks and has given rise to a wave of Greek pride and patriotism.
The mound is in ancient Amphipolis, a major city of the Macedonian kingdom, 100km (62 miles) east of Thessaloniki, Greece’s second city.
The wall surrounding it is 500m (1,600ft) in circumference, dwarfing the burial site of Alexander’s father, Philip II, in Vergina, west of Thessaloniki.