De Matteis is originally from Ostia, Italy. The small town is located near Rome, Italy, where she now resides. She is currently pursuing a degree in education at the Università degli Studi Roma Tre. She is also the owner and designer of a small clothing brand known as "Les Trois."
De Matteis has always had a passion for children. She currently volunteers for an organization known as Kim’s House. The organization helps children who are seriously ill from all over the world, particularly war-torn countries.
De Matteis entered the 2018 Miss Universe Italy pageant with hopes of representing her country on the Miss Universe stage and becoming the first Miss Universe Italy to bring home the Miss Universe crown. She represented the region of Lazio and competed against 11 other contestants for the title.
De Matteis was crowned Miss Universe Italy on November 11, 2018. After her win, De Matteis said the following.
"I am very proud and excited to be able to represent Italy in a great event like Miss Universe,” De Matteis said. “My goal will be to demonstrate that beauty is not only aesthetic beauty, but also intelligence, charisma and personality."
De Matteis will go on to compete for the title of Miss Universe 2018 on December 16, 2018 in Bangkok, Thailand.