The actress's alter ego Roz Fielding will bow out on screen in the summer after show bosses decided to write out the character, the News of the World reports.
Porter joined the cast of the soap a year ago and Roz soon caused a stir in the village as she played a key role in Holly Barton's descent into drug addiction.
Emmerdale's producer Stuart Blackburn commented: "The character of Roz was brought into Emmerdale for the purpose of Holly's addiction storyline, which has now concluded.
"Kirsty will therefore sadly be leaving us at the end of her current contract. She is a huge talent and an absolute professional. The door will be left wide open for a possible return in the future."
Meanwhile, Porter said: "I am looking forward to venturing on to new opportunities. I have loved every minute and had the pleasure of working with some wonderful and talented people."
Porter recently split from Emmerdale co-star Danny Miller, who plays troubled Aaron Livesy on the soap. She has previously appeared in Coronation Street, Doctors and Shameless among other programmes.
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