A 2-1 defeat away to Fiorentina was Sampdoria's third in a row and their seventh from 12 games so far this term.
After failing to win any of their opening seven Serie A games of the season, two consecutive victories against Livorno and Atalanta seemed to have turned the tide for Rossi.
However, a further defeat to Verona and a last-gasp loss at home to relegation rivals Sassuolo heaped the pressure back on the 52-year-old.
Youth coach Francesco Pedone will take charge of first-team affairs for the time being, with the entirety of Rossi's backroom staff also let go.
A Sampdoria statement read: "The club would like to thank everyone for their efforts and the professionalism demonstrated in these months of collaboration and wishes them the best fortunes for the rest of their careers."
Sinisa Mihajlovic and Zdenek Zeman are reportedly the early front-runners to replace Rossi at the Stadio Luigi Ferraris.
