Sammy, who plays village local Katie Sugden (née Addyman), said now is the time to "move on" after playing the role for 13 years.
She told Radio Times: "I'm going to miss everyone terribly. I've loved every second of playing Katie but no one becomes an actor to play just one role, so now it's time for me to move on.
"I don't want to give away anything about how Katie goes but it's certainly going to be dramatic."
Sammy joined Emmerdale in 2001 as Andy Sugden's girlfriend, and after a few affairs and a second marriage, she's now come full circle and married to Andy again.
However, with Andy's brother Robert causing problems for the pair, and with Katie intent on discovering Robert's affair, this gripping storyline is set to mark the end of Sammy's time on the ITV soap.
After Sammy's exit was confirmed, Emmerdale said in a statement: "Sammy Winward will leave Emmerdale in 2015 after playing Katie Addyman for 13 years. Her decision to pursue other acting opportunities, gives the writing team the opportunity to create a sensational, not-to-be missed storyline for her exit."
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