The tusk, believed to be of a Columbian mammoth, was measured at 8.5 feet long after researchers cleared enough dirt overnight to fully expose it.
Scientists with the Burke Museum of Natural History and Culture plan to remove the tusk sometime Friday evening using a construction crane at the site. They'll move it onto a pallet and use the crane to lift the pallet from the pit.
The tusk is between 20,000 and 60,000 years old and with the plaster encasing could weigh up to 500 pounds, said Christian Sidor, a paleontologist from the Burke Museum.
Once it's out of the ground, researchers will transport the tusk just a few miles away to its permanent home at the museum, on the University of Washington's Seattle campus.
The fossil eventually will go on display there. However, it's water-logged, and scientists say properly restoring and preserving it could take at least a year.
Construction workers found the tusk Tuesday about 30 feet below street level, thinking at first that it might be a pipe or a root. The company building a 118-unit apartment complex at the site has nearly stopped construction to accommodate the scientists.
No more fossils were found during the overnight dig, the museum said Friday.
"Generally tusks like these are the last thing left" after animals and time remove the bones and the rest of the creature, Sidor said.
The tusk's fate was entirely up to the landowner, who decided to donate it to the Burke Museum. Costs of the delay aren't known yet, said Scott Koppelman of AMLI Residential, which also owns apartment complexes to the south and west of the construction site.
Fossilized mammoth remains have been found numerous times in the Seattle area and across the state, so much so that the Columbian mammoth is the state's official fossil.
Still, most of the Burke Museum's collection is fragments. The tusk found this week would be one of the largest and most intact specimens found.
The museum's collection has 25 mammoth fossils from King County, including a tooth that was found a few blocks away from the tusk when the Mercer Street on ramp to Interstate 5 was built years ago.