Middlesex head coach Richard Scott has left the 2016 county champions after nine years in the role.
He took the job in 2009, helping them win Division Two in 2011 and the Division One title five years later, but they were relegated last year.
Ex-Middlesex seamer Richard Johnson, who was Scott's assistant, will take charge for the rest of the season.
"A process to recruit a new head coach for 2019 will commence immediately," said director of cricket Angus Fraser.
He told the club website: "As with every coach in professional sport we have shared highs and lows but to play a leading role in a side winning the County Championship is a triumph very few coaches can claim."
Johnson, 43, who played three Tests for England, will oversee the team's remaining seven Division Two matches, with former New Zealand all-rounder Daniel Vettori returning as head coach for the T20 Blast competition.
Middlesex are captained by England batsman Dawid Malan, with Eoin Morgan, Steven Finn and Toby Roland-Jones also on the staff - although Roland-Jones will not play again this season because of a back injury.
They are currently fifth in Championship Division Two.