The 27-year-old is quitting Roots Hall when his deal runs out and will seal his move to the Terriers once he has completed a medical.
"I can really see the ambition of the club and as a player that is exactly what you want to see," Clarke told Huddersfield's official website.
"It has been quite an easy decision to make, given the ambition and enthusiasm of the manager, the chairman and the staff around the club.
"I want to play at the highest level I possibly can, which is how it is should be, and given that the ambition of the club matches my own ambition the decision was an easy one.
"I can't wait to get started now; knowing exactly where my future lies has taken the weight off my shoulders. However, I don't know how fun the first week of pre-season will be."