Katherine will play Harriet Finch - a rival for Ashley Thomas as she wants to become the first woman vicar at St Mary's church in the village.
The actress told the Emmerdale website: "I am delighted to be joining the cast of Emmerdale. Harriet is great fun and I'm looking forward to seeing the character unfold."
Katherine played mother and teacher Sally Hunter in Hollyoaks for four years until 2005. Her other notable roles include appearances in the award-winning This is England and the first series of lottery drama The Syndicate. She has also made several one-off appearance in Coronation Street.
Harriet is a relative of village busybody Edna Birch, and arrives during a speed-dating event at The Woolpack where she and Ashley instantly clash.
She will appear on screen in Emmerdale from next month.
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