The 23-year-old led the team in the 2021 Royal London Cup on a temporary basis and will now take over the role permanently.
Haines gained further leadership experience this summer as stand-in captain for Sussex’s final four LV= Insurance County Championship fixtures. As previously announced, overseas player Travis Head will captain the four-day side next season.
Haines said: “I’m really excited. I felt like I got to grips with the role this year and I felt more comfortable the longer I did it. It’s great to be backed to do it again next summer.
“Whatever its position in the calendar, we’ll be taking the Royal London Cup extremely seriously. The club see it as an important part of its plans, and I want to win it and bring a trophy to Sussex.
“I was encouraged at my ability to do my job with the bat at the same time as leading the team last summer and I’m really looking forward to doing that again. Above all, I’m extremely proud to be a captain at my boyhood club.”
Sussex’s Championship & One-Day Head Coach Ian Salisbury added: “This is the next step for Tom as a cricketer and a person who has already developed hugely over the last 18 months. I was very impressed with Tom’s leadership last summer, which itself developed massively during the time he captained Sussex.
“The way he has matured is a great example for our other young players and that makes him the perfect man to lead the Royal London Cup side.”