The deal will see Finch remain at The 1st Central County Ground, Hove until at least the end of the 2018 season.
Twenty-one-year-old Finch, who was born in Hastings, began his Sussex career at the age of 13, and progressed through the Sussex age group and academy system.
He made his first-class and list A debuts in 2013 before making his first NatWest T20 Blast appearances a year later. Finch has subsequently sought to secure his place in the first-team, playing in all three formats during the 2016 campaign.
He scored his maiden first-class century this season in the university match against Leeds/Bradford MCCU before hitting a fine hundred in a tour match against a strong Pakistan attack.
Finch’s season, however, came to an abrupt end after sustaining a broken thumb on the opening day of the Specsavers County Championship match against Essex at Colchester.
Of his new contract, Finch said: "I’m absolutely delighted. I’m really looking forward to the next two years at the club. I can’t wait to get back out there and playing, and hopefully, winning some trophies in the next couple of years.
“It’s been nice to have a run in the first team this year, despite it being cut short due to injury. Hopefully I can continue over the next two years and cement my place in all formats.”
Sussex’s head coach Mark Davis said: “Harry has shown great improvement this year and we are thrilled to have him extend his contact.
“He is another young Sussex born and bred player who is showing considerable potential, and given opportunity will become an integral part of our squad.”
Finch didn't make a Sussex Premier League appearance for Priory this season during his increasing playing opportunities for Sussex, but did make seven appearances during their promotion-winning campaign of last summer, scoring 410 runs with a best of 126 not out.
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