Marcus Gray and two co-authors sued the pop star five years ago, claiming her 2013 track stole from their 2009 song Joyful Noise.
The nine-member jury in Los Angeles sided with Gray - and the next stage will be to determine how much the singer and other defendants will have to pay as a penalty for copyright infringement.
Monday's verdict represents a rare takedown of a pop superstar and her elite producer by a relatively unknown artist.
Gray's legal team had argued that the beat and instrumental line that featured through nearly half of Dark Horse were substantially similar to those seen in Joyful Noise.
Perry's lawyers argued that subjecting simple musical elements to copyright would hurt music and all songwriters.
"They're trying to own basic building blocks of music, the alphabet of music that should be available to everyone," her lawyer Christine Lepera said.
Perry and the song's co-authors testified that they had never heard of Joyful Noise or knew Gray previously. They said they did not listen to Christian music.
But Gray's lawyers claimed that Joyful Noise was widely disseminated and could have been heard by Perry and her co-authors - as it has millions of plays on YouTube and Spotify, and was on an album nominated for a Grammy.
Michael Kahn, who represented Gray, told the court that Perry had started her career as a Christian artist, adding: "They're trying to shove Mr Gray into some gospel music alleyway that no one ever visits."
In a light hearted moment during the proceedings, the singer offered to perform her song live when efforts to play Dark Horse to the courtroom ran into trouble.
Dark Horse was the third single from Perry's 2013 album Prism. It spent four weeks at the top of the Billboard Hot 100 and earned her a Grammy nomination.
She also performed the song during her half-time show at the Super Bowl in 2015.