The complaint was filed in Benton County on Tuesday against Barronelle Stutzman, owner of Arlene's Flowers and Gifts in Richland.
The lawsuit is in response to a March 1 incident where she refused service to longtime customer Robert Ingersoll. Stutzman did not return a call Tuesday night seeking comment.
Ferguson had sent a letter on March 28 asking her to comply with the law, but said Stutzman's attorneys responded Monday saying she would challenge any state action to enforce the law. Washington state voters upheld a same-sex marriage law in November, and the law took effect in December.
The state's anti-discrimination laws were expanded in 2006 to include sexual orientation.
Ferguson seeks a permanent injunction requiring the store to comply with the state's consumer protection laws and seeks at least $2,000 in fines.