Steve Clarke was sacked as West Brom manager on Saturday just hours after the team's 1-0 defeat at Cardiff.
The loss, which was their fourth in a row in the Premier League, left the Baggies two points off the relegation places.
"We have reluctantly come to the decision to relieve Steve of his duties after very careful consideration," said West Brom technical director Richard Garlick.
"It has been well documented that we have not had the rub of the green in certain games this season but that does not cloud the generally disappointing points return during this calendar year, culminating in today's fourth-successive defeat at Cardiff."
The 50-year-old Clarke, a former assistant coach at Chelsea and Liverpool, took over at West Brom in June 2012.
West Brom have won only seven of their 34 Premier League games during the calendar year although they did finish eighth last season, their best performance in the top flight.
Joint-assistant head coach Keith Downing will assume control of first-team affairs until a new full-time appointment is made, the club said in a statement.
"This club's track record proves we do not take such decisions lightly, having only enforced a change in this position three times in approximately 14 years," added Garlick.
"But with key games coming thick and fast, we felt it was important we acted now to give the club the best possible chance of a successful outcome this season.
"Our player wage bill is the highest in the club's history and we feel we have built a squad capable of being very competitive in the Premier League.
"We now begin the search to find a new head coach who will work within our existing structure, which has been in place for the past six years."
Clarke's team lost their season-opener 1-0 at home to Southampton at the Hawthorns.
But under his guidance, West Brom went on to celebrate a 2-1 victory against champions Manchester United, their first win at Old Trafford in 35 years.
Clarke admitted after Saturday's game that his team were not enjoying great fortune.
"We created some chances as we normally do, but at the moment things aren't quite going for us," he admitted.
"I don't think it's a lack of confidence. We tried to pass the ball but Cardiff started brightly and put us on the back foot.
"We resisted them and stayed in the game. The longer the game went on, the more we played and the more chances we created. It's difficult."
Clarke becomes the fourth Premier League manager to lose his job this season.
Paolo Di Canio was the first to go at Sunderland followed by Ian Holloway at Crystal Palace and Martin Jol at Fulham.