The mask was discovered over the weekend by a team of archaeologists excavating near the village of Topolchane, 290 kilometers (180 miles) east of the capital, Sofia. Its discovery, archaeologists said, indicates a Thracian king was buried in the tomb.
It was found together with a solid gold ring engraved with a Greek inscription and with the design of a bearded man in a timber-lined Thracian grave.
Team leader professor Georgi Kitov said that they also found a silver rhyton, silver and bronze vessels, pottery and funerary gifts.
"These finds confirm the assumption that they are part of the lavish burial of a Thracian king," said professor Margarita Tacheva, who was also on the dig.
"The artifacts belonged to a Thracian ruler from the end of the 4th century B.C. who was buried here," Kitov added.
According to Kitov, the Thracian civilization was at least equal in terms of development to the ancient Greek one.
In 2004, another 2,400-year-old golden mask was unearthed from a Thracian tomb in the same area.
Dozens of Thracian mounds are spread throughout the central Bulgarian region, which archaeologists have dubbed "the Bulgarian valley of kings" in reference to the Valley of the Kings near Luxor, Egypt, home to the tombs of Egyptian Pharaohs.