They have slumped to ninth in the League One table and were knocked out of the FA Cup by non-league Histon three weeks ago.
A statement on United's official website read: "The club have relieved Gary McAllister of his position as first-team manager.
"The board took the decision following the recent poor run of results that has seen the club slip to ninth in League One, 15 points behind the leaders and five points outside the play-offs.
"The decision was made to ensure that the new manager, when appointed, has the maximum number of games possible to move the club up the table and also before the transfer window opens in January to allow him to identify any new players he thinks can strengthen the current squad.
"The club would like to place on record its thanks for the loyal service given to Leeds United by Gary since joining the club as manager in January 2008 and wish him every success in the future.
"The club will make no further comment on a replacement until we are in a position to make an appointment, which we are committed to doing as quickly as possible."