The manager of the relegated side had received the backing of the board on the condition he got the Blues back into the top flight at first attempt.
McLeish quits Birmingham City amid rumours of Aston Villa link
Alex McLeish resigned as the manager of Birmingham City on Sunday following their relegation from the Premier League, with the club saying he had told acting chairman Peter Pannu in an e-mail he was leaving the Midlands side.
"Birmingham City Football Club can confirm Alex McLeish has resigned as manager," said a statement on the club's website.
"Acting chairman Peter Pannu received an e-mail resignation from McLeish earlier today (Sunday) while away on business in Amsterdam.
"No further comment will be made at this moment in time and updates will be released in due course," the statement added.
McLeish was immediately linked with a move to Birmingham's city rivals Aston Villa, still in the Premier League but without a manager following the resignation through ill-health of former France and Liverpool boss Gerard Houllier.
Birmingham were relegated on a see-saw final day of the season last month when a 2-1 defeat by Tottenham Hotspur, combined with other results, condemned them to a bottom three finish.
McLeish received the backing of the Birmingham board on the condition he got the Blues back into the top flight at the first attempt.
It was a rollercoaster campaign for the Blues, with McLeish also leading Birmingham to victory in February's League Cup final where Birmingham upset the odds to beat Arsenal 2-1 at Wembley — a result the manager labelled his "greatest achievement" in football.
McLeish, who made his name as a centre-half in Alex Ferguson's trophy-winning Aberdeen side of the early 1980s, took charge of Birmingham in 2007 but could not prevent the club being relegated.
The former Scotland international led Birmingham straight back to the lucrative Premier League and completed a 12-game unbeaten run during the 2009/10 season, a club record in the top division.