It ends the lengthy association the 34-year-old had with the Chelmsford-based club, who won the County Championship and Vitality T20 Blast this season with his help.
Essex confirmed Bopara, who made his debut in 2002, rejected "a new and improved contract" but added "the club wish him the best of luck in his future playing career."
Bopara told the official Sussex website: "I want to thank Sussex for this opportunity.
"It is a great chance to play under a respected and acknowledged coach like Jason Gillespie and with a very strong and talented team of players, many of whom will no doubt go on to receive international honours.
"On a personal note I have a burning desire to achieve greater things in this wonderful game. I will now put a greater emphasis on T20 cricket. I am fitter and stronger than I have ever been and playing my best cricket.
"I want to emphasise that I am not giving up red-ball cricket but I will, for now, play less of it to find time in the calendar year to up-skill in T20 and take my game to a new level as I still have a drive to play international cricket.
"Here's to the future and hopefully another decade of happy and successful times."
Bopara has 6,646 runs in 331 T20 appearances, making him the 19th highest run-scorer of all time in the format.
A switch to Division Two outfit Sussex looks set to allow the all-rounder the chance to take part in T20 competitions around the world, having previously appeared in the Indian Premier League and Big Bash League.
Sussex head coach Gillespie added: "I'm really, really excited about having Ravi Bopara join us at Sussex. It's a bit of a coup for us to be perfectly honest: what a wonderful player Ravi has been for England and Essex for a long, long time.
"His knowledge and experience are going to be invaluable in our dressing room. He's a great lad and we're very thankful that Ravi has decided to join us. Players and coaches alike are excited to work with him and learn from him."
In this season's Vitality Blast, Bopara had a batting average of 48.5 and a strike-rate of over 162. He also took 12 wickets and was named Essex's T20 player of the year.
