Tomas, born in 2012, has agreed terms running until 2028 and will join the Under-15 squad after progressing through Inter Milan's academy system, where he was one of the standout performers of last season, scoring 27 goals for the Under-14 side during a campaign that ended in a regional title.
The midfielder announced the move himself, posting on Instagram: "Happy and proud to be able to wear the colours I've always dreamed of. Forza Inter, always!"
He follows in the footsteps of his father, who joined Inter as a player in 1995 and went on to make 858 appearances across 19 seasons, captaining the club to the 2010 Champions League and five Serie A titles among 16 major trophies, before retiring in 2014 to become vice-president, a role he still holds.
Inter retired the number four shirt in his honour when he stopped playing.
Coaches within the club's youth setup have described Tomas as a technically gifted central midfielder with good vision and an eye for goal, comparing his playing style loosely to that of Henrikh Mkhitaryan, with an emphasis on composure on the ball and a willingness to get forward and support attacks.
He is considered physically underdeveloped for his age group by comparison with some peers, a gap coaches expect to close as he matures, though his early performances have shown no shortage of the competitive edge long associated with his father's playing career.
The move represents an early step in what the club hopes could become a second generation of the Zanetti name at Inter, even as those inside the organisation are careful to stress that any comparison to his father's achievements remains, for now, entirely premature given how far Tomas still has to go in his development.