The 33-year-old will play Danielle, a friend from Eli Dingle's (Joseph Gilgun) past who moves in with him after he almost runs her over outside the village.
The character, described as a "streetwise Londoner", will make her first appearance in September.
Stapleton said of her role: "Danielle is such an intriguing character because there's more to her than meets the eye.
"I'm really looking forward to starting filming and I think viewers can expect some interesting encounters as we explore the relationship between Danielle and Eli."
Emmerdale series producer Anita Turner added: "I'm so pleased to welcome an actress with Nicola's calibre to the show. Eli likes to manipulate people, but Danielle is a different proposition and she'll definitely get under his skin."