The paceman, who joined Sussex in 2012, has agreed to join Worcestershire for the 2018 season for their first season back in division one of the county championship following promotion.
Since making his debut in 2012 Magoffin has been an integral part of the team, taking 340 wickets in 89 appearances for Sussex.
Magoffin has led the pace attack throughout his time at Sussex, finishing with more than 50 County Championship wickets in five of his six seasons with the club. In 2014 he finished the season as leading wicket taker within division one with 72 wickets to his name.
Sussex bosses said that despite experiencing a frustrating final season because of injury, Magoffin has continued to be a leader within the dressing room helping nurture the young and exciting pace attack.
Alongside this he has been an ever-present figure within the wider Sussex community helping the Sussex Cricket Foundation with numerous activities and community events during 2017.
Sussex are well placed for life without Magoffin with Chris Jordan and Jofra Archer set to lead the pace attack across all formats, ably supported by a strong group of young bowlers who will continue to gain vital in match experience.
Magoffin said: “I’d like to thank the members, supporters, board, office and professional staff for an absolutely amazing six years at the club. The tremendous support and friendship of all my team mates, current and past, has been fantastic and I wish the current squad all the best for the forthcoming seasons.
“In particular I would like to thank Mark Davis, Jon Lewis and Mark Robinson. Robbo originally brought me to the club for a six-week period, which has ultimately turned into the best 6 years of my career. Without their support, this would not have been possible, for which I am truly grateful.
“I am disappointed not to be able to continue my career at Sussex, however, moving back to Worcestershire where I played 10 years ago is the right opportunity for my family and I. My wife’s family is based in Worcestershire and it will be great for the kids to be closer to them all.
“I have had the time of my life at Sussex and I look forward to following the team’s future success as a supporter of this great county.”
Keith Greenfield, Sussex Director of Cricket, said: “Steve has been a fantastic servant for the club, he has led our attack with great skill over the past 6 years.
“This year has been an extremely difficult year for him, but he has continued to be the ultimate professional, and we wish him well with his new challenge.”
Sussex finished fourth in division two of the championship as Worcestershire won the title and Notts joined them in going up to division one.