Wednesday made the announcement late on Monday.
"The performances and results have not been of the level expected since Tony Pulis was appointed," chairman Dejphon Chansiri said in a statement on the club website.
"There are also other issues which have had a bearing on this decision."
Wednesday said first-team coach Neil Thompson would take charge of the side until a permanent appointment was made, starting with Tuesday's game against Middlesbrough.
Wednesday lie second bottom of the table on 13 points, three from a position of safety.