The 28-year-old was an ever-present in the side which won the County Championship title in 2016.
"He is one of the finest cricketers in the country," managing director of cricket Angus Fraser said.
"As a glove-man there are few better than him and he is capable of producing match-changing innings with the bat."
Since making his debut for Middlesex in June 2009, Simpson has made almost 250 appearances in all formats.
The left-hander has scored 6,687 runs for the club and has 472 dismissals to his name.
Middlesex wicketkeeper-batsman John Simpson has extended his contract, keeping him at Lord's until the end of the 2019 season.