The 20-year-old batsman, who also keeps wicket, is a product of Sussex’s player pathway and has represented the county since the age of ten.
A former pupil of Bede’s School in Eastbourne, Orr has already played 15 times for the 2nd XI since his debut in 2018, top-scoring with 85 not out against Yorkshire in a three-day match at the end of the 2019 season.
Playing for Loughborough University against Leicestershire last week, Orr top-scored in the students’ first innings with 60.
He becomes the tenth pathway graduate in Sussex’s current professional squad and is the fourth of those to sign their first contracts ahead of the new campaign, alongside Jamie Atkins, Jack Carson and Henry Crocombe.
Orr said: “I am so excited to sign my first contract with Sussex as it has always been an ambition of mine since joining the age group squads when I was ten.
"It means so much to know that the coaches as well as the directors have belief in my game to try and take it to the next level.
"Sussex means so much to me as I’ve been not only been playing for them but also supporting them for so long, so to finally have an opportunity to be a professional cricketer really is a dream come true.
“My goal for the season is to make my first-class debut, and then moving on from that try and cement my place in the side in the long term.
"I enjoy all formats of the game and want to improve myself to try and get an opportunity in all formats of the game as well.
"In the end my overall goal in the long term is to play for England.”
Championship and one-day head coach, Ian Salisbury, added: “Ali epitomises our strategy of producing homegrown players for Sussex and, hopefully in the future, for England.
"He is hardworking and enthusiastic and comes in every day trying to get better and learn from those around him.
“He has impressed us every time he has been around the squad. Even since starting university, Ali hasn’t stopped coming in and training hard.
"He is a fantastic example to our pathway players and is a testament to the culture and environment created by Richard Halsall (Sussex Cricket’s pathway director) in the youth set-up.
“Ali can bat anywhere from one to seven and is a keeper as well, so provides us with plenty of options and cover.”
James Kirtley, T20 head coach, said: “Ali is another player who over the course of the last three or four years who we’ve had an eye on.
"Obviously opportunities were limited in 2020, but he performed extremely well in 2019 and he has all the values we are looking for in our young players.
“He is a talented batsman who also gives us cover in the ‘keeping department.
"He gave a hint of what he’s capable of with good runs against a first-class attack at Leicester last week and he fits in well with our strategy here at Sussex. I’m sure he has a bright future in the game.”