The deal makes the 22-year-old available predominantly for the white ball portions of the season while allowing him to complete his studies at Cardiff Metropolitan University.
The left-handed batter and part-time off-spinner made his first team debut this summer, appearing in three first-class, two List-A and six T20 matches, including Sussex Sharks’ Vitality Blast semi-final at Edgbaston.
Born in Oxford, Ward is a product of the talent development partnership between Oxfordshire and Sussex. He is one of two players – alongside James Coles – to have followed this route to the professional squad at The 1st Central County Ground.
Ward said: "I’m delighted to have signed my first professional contract with the club that I’ve been involved with since the age of 15.
"I want to thank the club for giving me this opportunity and, with a winter behind me, hopefully I can contribute to winning games of cricket for Sussex next season.
"We have a good group of young players and, if we work hard, hopefully we can build something special at the club.
"I would also like to thank my family for supporting me through the various lockdowns when I wasn’t contracted, and also Oxfordshire cricket for their support with the partnership with Sussex.”
James Kirtley, Sussex’s T20 head coach, added: "Harrison showed some of his ability at Finals Day with the bat with some very strong shots.
"He is a more than capable off spinner a very fine fielder. Harrison delivered some great skill last summer which allowed him to force his way into the T20 side.
"At Sussex we understand that not every player will make the traditional journey from being a four-day player and then adapting to limited overs cricket.
"Harrison is at the opposite end of the spectrum, and I personally think it is a sign of great foresight that we understand that players can mature in different ways and that all types of players deserve that opportunity.”
Sussex Championship & One-Day head coach, Ian Salisbury said: "Harrison was around the squad a couple of years ago and things didn’t quite work out for him, but since then he’s shown real determination and worked extremely hard to earn another opportunity.
"We saw glimpses of what he could be capable of last season, and he’s made it clear through his approach that he is absolutely committed to building on those and making it as a pro cricketer.
"We want to build a squad of multi-format players, but there’s more than one way of achieving that. Harrison’s skills currently lean towards the white-ball game, but you just have to look at a player like Davey Warner to see that that’s no barrier to ultimately achieving success in the longer form. Our aim as coaches is to help Harrison do that."