The relic from the ice age was found in Seattle, Washington, as constructors excavated the earth to begin laying the foundations for a 118-flat building.
The tusk, over eight feet in length, is believed to be from a Columbian mammoth which inhabited the North American continent.
Ancient relatives of elephants, the mammoths became extinct as the glaciers began to recede at the end of the ice age between 10,000 and 11,000 years ago.
The tusk was found about seven feet below the ground at the construction site and will undergo carbon dating so scientists can pinpoint an exact age for it.
A statement from the Burke Museum of Natural History and Culture said: "The tusk represents a rare opportunity for palaeontologists and other researches to understand the paleoenvironmental conditions present in Seattle during the ice age."
“The scientific and educational benefits of this discovery clearly outweigh the costs and delay”Scott Koppelman
Scott Koppelman, the vice president of the company building the apartment block, revealed the amazing discovery will cause a significant delay to his project.
He added: "But the scientific and educational benefits of this discovery clearly outweigh the costs and delay.
"This is an exciting discovery for our local Northwest history."
Although one of the photos appears to show the rare tusk wrapped in tinfoil it, it is in fact a special plaster meant to protect the gigantic tusk while it is moved.
Columbian mammoths grew up to 12 feet in length and were about the size of a modern Asian elephant.