The 29-year-old is currently on a season-long loan at Chelmsford, having made the switch on the eve of the campaign having been relieved of the captaincy at Northants.
Since joining Essex, Rossington has averaged just under 30 in red-ball cricket whilst also producing some powerful ball-striking at the top of the order in the Vitality Blast.
“I’m really pleased to sign a deal with Essex and commit my future to the club," said Rossington, who signed for Northants from Middlesex in 2014.
“Since joining on loan a few months everybody at Chelmsford has made me feel extremely welcome.
"It’s a great dressing room to be involved in and the amount of talent and determination in there is second to none.
“The members and fans have also been great. The support we’ve received during my short time here so far has been amazing and hopefully we can repay that with more victories and some silverware come the end of the season.”
Rossington, who was part of the Steelbacks team that won the 2016 T20 Blast, and also captained the Championship promotion team of 2019, also thanked the fans at Northamptonshire for their backing while he was at the club, as wellas his former team-mates.
“Whilst I’m keen to continue my journey with Essex and excited about the future, I’m very grateful for my time with Northamptonshire," he said.
"I would like to thank all of the brilliant fans for their consistent support over the years.
“And to my team-mates at Wantage Road, it’s been a pleasure to have shared a dressing room with you."
Essex Head coach Anthony McGrath is delighted to have secured the services of Rossington.
“Adam has been a great addition to the dressing room and has fit into the team seamlessly," he said.
“He’s done a good job for us with both the bat and the gloves in all formats and we’ve also benefitted from his leadership qualities and experience.
“Securing him for three years is a great coup for the club and hopefully he can help us push on from our decent start to the campaign going into the business end of the season and beyond.”