The 38-year-old left-handed opener, who retired from international cricket in 2018, played his entire county career with Essex.
His contract at the county expired at the end of the domestic season last month and Cook, part of Essex's County Championship-winning side in 2019, has elected against seeking a renewal.
"It is not easy to say goodbye. For more than two decades, cricket has been so much more than my job," he said in a statement on the Essex website.
"It has allowed me to experience places I never dreamed I would go, be part of teams that have achieved things I would never have thought possible and, most importantly, created deep friendships that will last a lifetime."
"From the eight-year-old boy who first played for Wickham Bishops Under 11s to now, I end with a strange feeling of sadness mixed with pride. Although, above all, I am incredibly happy."
Scorer of 12,472 test runs for England and capped 161 times, Cook said it was the right time to go and "make way for the new generation to take over".
Cook was England Test captain from 2012 to 2017 and also captain for 69 one day internationals from 2010-14.
Cook bowed out of England duty with a hundred against India at the Kia Oval in September 2018 and was given a knighthood for services to cricket a few months later.
He scored 766 runs in seven innings during the 2010-11 Ashes tour to help England to their first series win in Australia for 24 years.
with PA
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