The 29-year-old actress announced she was leaving shortly after filming scenes that will go out over Christmas, which sees her character tie the knot with Andy Sugden.
Her shock departure is seen as a huge loss for 'Emmerdale' because show insiders believe she has helped turn the soap into a ratings hit, regularly beating 'EastEnders'.
A show source told The Sun: “Sammy is the Michelle Keegan of Emmerdale so everyone’s gutted she’s leaving.
“She’s hugely popular and her character has been a big part in the show overtaking EastEnders.”
It's still to be decided how the character will leave the show next year, but the actress is said to be keen for Katie not to be killed off.
“She wants to spread her wings and try theatre work and other telly. Emmerdale is virtually everything she has ever known.”
Sammy landed the role of Katie in 2001, when she was still studying for her GCSEs.