Saelemaekers joined Milan initially on loan from Anderlecht in January 2020 and made that move permanent eight months later.
The 22-year-old has so far made 64 appearances across all competitions for Stefano Pioli’s men, scoring four goals.
"I'm happy to have grown with this team," Belgium international Saelemaekers told Milan's official website, before expanding on his official Instagram account.
"Salamandra remains! I'm delighted to continue my story with this great club that has welcomed me and made me feel important immediately," a post read.
"It's a wonderful emotion that I want to share with all the people who have accompanied me on this journey and in particular with all the Milan fans who are special.
"I'm proud of my growth which has gone hand in hand with the team. Now, the target is always the same: work, grow and win!
"Forza Milan God is Great!"
Saelemaekers has won five caps for the Belgium national team, scoring one goal.
AC Milan, who are second on the Italian Serie A table, entertain Hellas Verona later on Saturday.
