The promising youngster has signed a deal until 2018 and is determined to make a name for himself in the years to come
The 18-year-old is regarded as one of the San Siro side's biggest attacking talents and penned a five-year deal with the Rossoneri on Wednesday.
"Today was one of the best days of my life. I have to thank Adriano Galliani and my agent for this contract and I hope to stay at Milan for a long time," Petagna told Milan Channel.
"I made my debut in the Champions League against Zenit, personally 2012-13 was a wonderful season for me. The faith from coach Massimiliano Allegri is important and I want to repay that in the best possible way next season.
"I have to improve especially in terms of movement in attack and movement in the area. Galliani has spoken well about me and he said I have to be a professional on and off the pitch."
Petagna has made one official appearance for Milan so far.