Born on November 15, 1996, in Hautrage, Belgium, (Son of Houcine Amallah, a Moroccan-origin amateur coach, from Nador, and Antoinette Caufriez, a Belgian and Italian descent homemaker), Selim Amallah grew up in the suburbs of Mons in the Hainaut province. Selim has a sister and a brother named Mohamed, who is also active in amateur football. Although his mother is of Italian descent, Selim grew up in a Moroccan culture and regularly visited his paternal family in Nador, traveling there for at least one month each year.

Selim started playing football at the age of four, enrolled by his father at RAEC Mons, where his father was a youth team coach. He later joined the youth academy of RSC Anderlecht, where he was noticed for his impressive technical performances. However, despite his evident talent, Selim faced challenging times, including the tragic death of his mother in 2011. After a brief stint at Anderlecht, he returned to RAEC Mons, where he wore the number 21 jersey in honor of his mother. In 2015, Selim Amallah signed his first professional contract with Royal Mouscron-Péruwelz, marking the beginning of his professional football career.