According to an email from lawyer Burt Boltuch, the suit will be filed Wednesday morning in Oakland, Calif., at the Superior Court of Alameda County and a press conference will be held afterward to discuss the allegations, the Contra Costa Times reported.
The suit accuses Ellis of sending "dozens of pictures" of "flagrant sexual contact" to a then-Warriors female employee between Nov. 2010 and Jan. 2011.
Boltuch said the accuser had photographic evidence of the harassment. He said both the accuser and the photos will be revealed at the press conference.
"You will be shocked when you see it," Boltuch said. "It's pretty disgusting and graphic.
The Warriors are being accused of covering for Ellis, with Boltuch claiming the plaintiff was let go because the team "wanted to cover it up."
Boltuch also claims the Warriors have refused to resolve the matter previously.
A Warriors spokesman said neither the team nor Ellis had a comment and they had not received the lawsuit, Yahoo! Sports reported.
Ellis, 26, was selected by the Warriors in the second round of the 2005 NBA Draft. He averaged 24.1 points and 5.6 assists in 80 games last season.