The Czech Republic full-back was a key part of the Albion under-23 side that won promotion via the Premier League 2 Division 2 play-off final last season, and will now link up with the Italian side ahead of the 2018/19 campaign.
Brighton U23s' coach Simon Rusk said, “This is a good move for Ales, and an exciting step in his career.
“Moving to Italy will allow him to experience a different kind of football at first-team level, and will help him to develop as a player.
“He was part of the success we achieved last season and this move is a reward for his contribution."
Mateju made 15 appearances in Premier League 2 last season, and featured for the first team in the Carabao Cup matches against Barnet and AFC Bournemouth.