Paleontologists with the University of Washington hope to move the tusk to the Burke Museum of Natural History and Culture in time for public viewing at its annual Dino Day on March 8.
It's the latest example of the state fossil: the Columbian mammoth, Mammuthus columbi.
"They are very rare," said Christian Sidor, curator of vertebrate paleontology for the museum on the Seattle campus. "It will be an exciting fossil to get back to the museum."
A Transit Plumbing employee discovered the tusk Tuesday at a construction site.
Worker Joe Wells "actually uncovered it with a shovel and sort of figured it was a tusk and stopped and contacted the general contractor," Transit Plumbing owner Jeff Estep said.
When Estep was told about the discovery, "I was going, like, 'Yeah, right.'"
It was more exciting than the time they found a jar of money someone had buried in a backyard, he said.
The Burke Museum was waiting for approval from the property owner Wednesday to excavate the site and perhaps see if there are more buried bones.
"When you start digging, you never know," Sidor said.
Millennia before Seattle was founded, prehistoric animals wondered around what is now Lake Union and the lands covered by the university and the headquarters for online retailer Amazon.com.
Construction has been constant in recent years in the South Lake Union neighborhood, which also features biotech companies and apartments and condos for thousands of workers.
Similar tusks from the extinct relative of the elephant have been found in Washington, and a tooth from a mammoth even was found at a construction site at the university.