Cassar, 43, was 38-37-30 in his 3-plus seasons with Real Salt Lake, including an 0-2-1 mark this season with just one goal scored. Real is currently on a 11-match winless streak dating back to last season.
Cassar led RSL to the MLS playoffs in two of his full three seasons at the helm, losing in the first round in 2016, and the conference semifinals in 2014.
Goalkeeping coach Daryl Shore will take over interim head coach duties while the search for a replacement begins. The rest of the RSL technical department will remain on staff.
Cassar was given a 3-year contract extension in November of last year, but the team had options on the final two years.
"At the conclusion of last season, we felt that while we were disappointed in the end result, Jeff had done enough to earn another shot to lead the team in the 2017 campaign. We have a high sense of urgency for progress this season, and have made many off-season changes towards that endeavor. So, while these decisions are always difficult, we, as an organization, felt that it was necessary to make the change in our head coaching position now. The process for a hiring new head coach to lead the club has already begun. We will work quickly and diligently to ensure that we find the right leader that shares our values, passion, commitment and vision."
RSL next takes on the New York Red Bulls on Saturday.
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