The fist-sized lump of precious metal weighs more than 6lb (2.8kg) and is expected to carry a price tag of $350,000 (£220,000).
The chunk is known as the Butte Nugget, after the Northern California county where a gold hunter says it was found in July.
The owner, who wishes to remain anonymous, said it was mined on public land in a mountain range.
But he asked dealer Don Kagin to keep his name and the exact location of the discovery secret.
David McCarthy, a rare minerals expert, told the San Francisco Chronicle he was escorted blindfolded to the site to check it was genuine.
Mr Kagin said he had taken pains to establish the veracity of this find after he was hoodwinked in 2011 when another nugget purportedly unearthed in Sierra Nevada, California, turned out to have come from Australia.
That seller was trying to inflate the price of his land where he falsely claimed the 8.2lb chunk, which sold for nearly half a million dollars, had been discovered.
The Butte Nugget is being unveiled at the San Francisco Fall Antiques Show.
Butte County was also where the 54lb Magalia Nugget, the largest in the world at the time, was mined during the original California Gold Rush in the mid-1800s.
