Haynes, 22, who came through Worcestershire's academy and had been capped by England Lions, was out of contract and has agreed a three-year deal at Trent Bridge.
A highly rated top-order batter, Haynes has impressed across formats since his first-team debut in 2019. He averages 34.91 in first-class cricket, with four hundreds, 42.63 in 11 List A appearances and has a T20 strike rate of 143.31.
"Jack is already a good player though it his hunger to move his game forward that really excites me," Nottinghamshire head coach, Peter Moores, said. "He has already gained valuable experience in red and white ball cricket, which gives him a great platform to build on going forward,"
"As a top-order batter in all formats he will strengthen our batting unit as well as adding healthy competition. We pride ourselves in helping players fulfil their potential and we have already seen many of our players move their games forwards at Trent Bridge.
"We pride ourselves as a coaching team in creating opportunities for all our players to keep improving, knowing that as they improve, so do Notts. Helping players develop is one of the most exciting parts of coaching in my opinion."
Haynes made his England Lions debut in Sri Lanka over the winter and has also played for Oval Invincibles in the Hundred.
"I'm really excited to move to Nottinghamshire," Haynes said. "Trent Bridge is always a place I've loved coming to as a player; the club has a great feel about it, and the facilities and pitch are conducive to helping me develop as a cricketer.
"When I sat down with Pete Moores and Ant Botha, they were both passionate about the direction of this squad, and how I can fit into it. I'm under no illusion that it'll take hard work to nail down a place in the side and continue to grow in all formats, but Notts have a track record of taking on players and improving them to help them reach their potential."
Worcestershire's chief executive officer Ashley Giles said: "It is always disappointing to see a player nurtured by our own pathway and academy leave the club. We would like to take this opportunity to wish Jack every success in the future."