The France striker, who scored 39 goals in 119 appearance for the Blues, won the Champions League, Europa League and an FA Cup during his three-and-a-half-year spell at Stamford Bridge.
However, his playing time was limited under Thomas Tuchel with Timo Werner and Kai Havertz often selected ahead of him last season.
Now, Giroud could form a formidable attack with Zlatan Ibrahimovic, who is recovering from a knee injury.
Notably, the 34-year-old has been given the 'cursed' No.9 shirt at AC Milan.
The number has been worn by prolific strikers such as Gonzalo Higuain, Luiz Adriano, Fernando Torres and more since AC Milan legend Filippo Inzaghi left the club in 2012.
The Italian was the last striker to regularly score while wearing the No.9 for AC Milan.
The No.9 'curse' since Filippo Inzaghi's retirement
Alexandre Pato: 2 goals in 7 appearances
Alessandro Matri: 1 goal in 18 appearances
Fernando Torres: 1 goal in 10 appearances
Mattia Destro: 3 goals in 15 appearances
Luiz Adriano: 6 goals in 29 appearances
Gianluca Lapadula: 8 goals in 29 appearances
Andre Silva: 10 goals in 40 appearances
Gonzalo Higuain: 8 goals in 22 appearances
Krzysztof Piatek: 5 goals in 20 appearances
Since then, NINE forwards have struggled for form with the iconic number on the back of their shirts.
Torres scored just once in 10 matches, Higuain netted eight in 20, while its most recent occupier Krzysztof Piatek managed just five in 20.
Giroud will look to aid AC Milan's title challenge after they finished second and 12 points behind rivals Inter last season.