The South African plays for Essex on a Kolpak deal and has taken 131 County Championship wickets in his two seasons with the club so far.
The off-break bowler, 29, said: “From the moment I arrived two years ago I was made to feel part of the group and at home in Chelmsford. It is now a place I now consider my second home, so it was an easy decision for me to make.”
Alongside fast-bowler Jamie Porter, Harmer was instrumental in helping Essex to win the County Championship Division One in 2017.
But this year they finished third behind champions Surrey and Somerset.
“We tasted success with the County Championship win in 2017 and it hurt not winning it again last year, so our goal for 2019 has to be to reclaim the title,” said Harmer.
Essex’s Head Coach Anthony McGrath said: “Harmy is a world-class bowler who can do serious damage with the ball.
“He’s taken over 100 first-class wickets in each of the last two calendar years, which in itself is an unbelievable achievement. He’s such a vital part of the bowling attack, so I’m thrilled he’ll be here for the foreseeable future.”
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