Ahead of the current match against Yorkshire, the opening batsman was the fourth highest run scorer in the LV= Insurance County Championship, with 622 runs from his 13 innings so far this season.
Haines’ century against Northamptonshire in Sussex’s most recent fixture was his second of the campaign and the fourth of his professional career.
The 22-year-old has played 28 matches since making his first-class debut for Sussex as a 17-year-old in 2016.
Reacting to his extended deal, Haines said: “I’m delighted to have signed a new contract with my home club. There’s a really exciting feel around the club and I’m extremely proud to extend my contract here.
“Hopefully in the next few years we can grow stronger as a team and start challenging for silverware in all forms. There’s a lot of promise in this squad and we’ve got good support from our coaching staff to improve day in, day out.”
Sussex’s Championship and One-Day Head Coach, Ian Salisbury said: “Tom fully deserves his new contract. Since the last game of last season at The Oval, he’s scored three centuries and has shown that he understands what it takes in terms of training and preparation to be a successful opener in England. As much as anything he’s showing the mental application required to make himself the best player he can be.
“He’s continuing to develop and that was shown in his hundred in the second innings against Northants last week, which I think was his best knock to date. Tom’s hungry not just to score runs for Sussex but to learn how to make the step up to the next level.”
Sussex’s Performance Director, Keith Greenfield added: “We are determined to enable our young talent to thrive for Sussex and push on for England honours in the future. Tom is showing everyone his undoubted quality near the top of the list of leading run scorers in the country and he knows he is going to be backed by everyone here at the club. We are thrilled that he has committed to a new long- term contract with his home county.”