Jason Gillespie, Yorkshire's head coach, will be leaving the club at the end of the 2016 season to return to his native Australia.
Gillespie, who was appointed in November 2011, helped guide the club from the second division to back-to-back Championship titles in 2014 and 2015, with the club firmly in the hunt for a third title following last week's victory over Nottinghamshire at Scarborough.
In the course of 76 Championship fixtures at the helm, he suffered just five defeats.
Yorkshire had hoped that Gillespie would stay for at least another year, and he had intended to reflect on the matter for a few months upon returning to Australia at the end of the English season. But after defeat to Surrey in the Royal London Cup semi-final on Sunday, he advised the Yorkshire board of his decision.
The announcement, which had been anticipated for much of the season, comes after Gillespie took up a contract to coach the Adelaide Strikers in the Big Bash, and his wife, Anna and their four children have recently returned to live back in Australia.
"The club would like to place on record its thanks to Jason," read a club statement. "The focus will now be very much on the remaining four County Championship fixtures, beginning with Wednesday's trip to the Ageas Bowl to face Hampshire, and on securing the first Championship treble seen at Headingley since the 1960s."
Martyn Moxon, Yorkshire's director of cricket, will not begin the search for Gillespie's replacement until the end of the current season.