Chairman Richard Murray said in a statement on Saturday evening, "On reflection of results so far this season, and looking at the future challenges for the remainder of the campaign, it was agreed by both parties that Alan would leave the club with immediate effect."
Pardew took over on Boxing Day 2006 and almost saved the club from Premiership relegation until Tottenham ended the dream in May 2007.
Given funds to build his own side to challenge for promotion in the second tier, Pardew brought in players such as Luke Varney, Chris Iwelumo and Paddy McCarthy and after a pitiful end to the season, they finished the season in 11th.
This season started well after picking up wins in their first two home games, including a superb 4-2 victory over newly relegated Reading but high scoring home defeats to Barnsley and United yesterday put the nail in the coffin for Pardew.
He leaves the club after 23 months in charge, winning 28 games out of 90.
Former assistant to Pardew, Phil Parkinson takes temporary charge of the side for Tuesday's trip to Queen's Park Rangers.
For Murray and the board, the hunt for a successor begins.