Andre Villas-Boas has "parted company" with Chelsea, the club have confirmed.
The 34-year-old lost his job less than 24 hours after Saturday's Barclays Premier League defeat at West Brom, which left the Blues fifth in the table.
Chelsea also announced assistant manager Roberto Di Matteo would take charge of the team until the end of the season.
Villas-Boas was only appointed eight months ago and cost billionaire owner Roman Abramovich £13.3million to land from Porto.
Chelsea said in a statement on their official website, www.chelseafc.com: "Andre Villas-Boas has parted company with Chelsea Football Club today.
"The board would like to record our gratitude for his work and express our disappointment that the relationship has ended so early.
"Unfortunately the results and performances of the team have not been good enough and were showing no signs of improving at a key time in the season.
"The club is still competing in the latter stages of the UEFA Champions League and the FA Cup, as well as challenging for a top-four spot in the Premier League, and we aim to remain as competitive as possible on all fronts.
"With that in mind, we felt our only option was to make a change at this time.
"With immediate effect, Roberto Di Matteo has been appointed first-team coach on an interim basis until the end of the season."