Alejandro Gomez scored a stunning opener, nutmegging Andrea Conti before firing home, and Atalanta’s final three goals came in the last half hour.
Mario Pasalic scored and Josip Ilicic netted twice before Luis Muriel struck.
Milan had not lost by five goals in any competition since being beaten 5-0 by Roma in May 1998.
On Friday,
FT Fiorentina 1 – 4 Roma
On Saturday,
FT Udinese 2 – 1 Cagliari
FT Inter 4 – 0 Genoa
FT Torino 1 – 2 SPAL
Yesterday,
FT Atalanta 5 – 0 Milan
FT Parma 1 – 1 Brescia
FT Lecce 2 – 3 Bologna
FT Sassuolo 1 – 2 Napoli