The mosaic - 3m (10ft) wide and 4.5m (15ft) long - depicts a man with a laurel wreath driving a chariot drawn by horses and led by the god Hermes.
The burial site is said to be the largest ever found in Greece.
It dates from the late 4th Century BC, spurring speculation that it is linked to Alexander the Great of Macedon.
Archaeologists started digging in August and think the magnificence of the tomb means it was built for someone very important.
Some observers say the tomb could belong to a member of Alexander's immediate family - maybe his mother, Olympias, or his wife, Roxana - or another Macedonian noble.
Others believe it could be a cenotaph, a monument built in honour of a person whose remains are elsewhere.
The discovery of the floor mosaic was announced by Greece's culture ministry on Sunday.
It says Hermes is depicted as the conductor of souls to the afterlife.
The image is made up of pebbles in white, black, blue, red, yellow and grey.
Amphipolis site
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.
Alexander the Great
Archaeologists unearthing a huge ancient burial site at Amphipolis in northern Greece have uncovered a large floor mosaic.