The actress, who plays Hayley McCrone on the BBC Scotland show, will today (April 27) film her final scenes.
Viewers have seen Byrne on screen as Hayley for the past three years.
Byrne told the Daily Record: "It wasn't my decision to go and I'm really sad to be leaving, but I have a theatre show coming up right away."
A River City source added: "People on River City know there are going to be more cuts with a 20% budget clawback to be met."
River City fans will see Hayley's exit in July, while Byrne will soon be appearing in the musical Company at Glasgow's Cottier Theatre.