The West Brom board made the decision following the club’s poor run of results, with 13 losses on their last 18 games in all competitions.
West Brom’s first-team coach Michael Appleton will take control of first-team affairs whilst the club are looking for a new manager.
Chairman Jeremy Peace said: “This has been a tough decision but we, as a Board, believe it is the right one to give the Club the best possible chance of remaining in the Barclays Premier League.
“Our poor sequence of results stretches back more than three months and there has been little sign of it coming to an end.
“If this run continues much longer, achieving our goal of retaining our Premier League status will become increasingly difficult. That is why we felt compelled to act now.
“This Club’s track record proves we do not take such decisions lightly, with Roberto being only our fourth Manager or Head Coach in almost 11 years.
“We now begin the search to find a new Head Coach who will fit into our existing set-up and work under our Sporting & Technical Director, Dan Ashworth, with a view to taking the Club forward.
“On behalf of the Board, I would like to thank Roberto for his efforts over the past 19 months, particularly for his contribution towards last season’s promotion success and our very promising start to the current campaign. We wish him and his family well for the future.
“Roberto embraced the structure in which we wanted him to work and he has been a good colleague.
“However, we are in a results-driven industry and felt we had no choice but to act now.”