The Steelers have already confirmed the news and announced that the Seahawks will receive a fifth-round pick in 2023 from Pittsburgh for Witherspoon.
The Steelers also released DL Henry Mondeaux.
Witherspoon had fallen out of favor in Seattle and was behind D.J. Reed at corner.
Witherspoon, 26, is a former third-round pick of the 49ers back in 2017. He played out the final year of his four-year, $3.88 million contract that included a $973,572 signing bonus.
The Seahawks signed Whiterhspoon to a one-year contract worth $4 million this past March.
In 2020, Witherspoon appeared in 11 games for the 49ers and recorded 20 tackles, an interception and four passes defended.