Tesla Motors and Panasonic officially agreed to partner on the construction of the Gigafactory, the massive plant expected to crank out lithium-ion batteries for as many as 500,000 electric vehicles by 2020.
A final deal was widely anticipated amid media reports in recent days, but Musk also said it was as good as done back in May.
Under the agreement, Tesla will handle the land, buildings and utilities, while Panasonic will invest in the necessary equipment and manufacture the lithium-ion cells.
The location of the factory is still unknown, though Arizona, California, Nevada, New Mexico and Texas were all in the running in recent months. Tesla may provide additional details after market close on Thursday, when it hosts an investor call to discuss quarterly earnings results.