Redknapp took charge at Birmingham City on a short-term deal in April and kept the Blues up with two wins in their final two games of the season.
But the 70-year-old endured a shocking start to the new season, losing six of their opening eight Championship matches.
Redknapp embarked on a rebuilding programme at City over the summer, which included the club-record signing of Jota, brining in 14 new players.
But today’s latest defeat to Preston, which kept them drop into the Championship relegation zone, was the final straw for the Birmingham hierarchy.
Birmingham took the lead at St. Andrew’s Stadium through Colin Maxime in the first half but capitulated after the half-time break.
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Daniel Johnson, Jordan Hugill and Tom Barkhuizen all scored for Preston as Alex Neil’s side came back to win 3-1.
Lee Carsley, who arrived at Birmingham under Redknapp in June, will take charge of first-team duties for the time being.
Birmingham tweeted after the defeat: “Blues have parted company with manager Harry Redknapp.
“Lee Carsley, Blues Head Professional Development Coach, will step in as Caretaker Manager.”
After the defeat to Preston, Redknapp said: “We have got to lift ourselves. There is a team waiting to gel but we have suffered several injuries to key players.
“Fans deserve better and that was my motivation to carry on this season.
“I do not come to work because I need the money, I enjoy the job and leaving home at 5.15 every morning. I would love to be the manager to bring some success to Birmingham.”