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Di Guglielmo, a full Italy international, arrives at Roma Women from fellow Serie A side Empoli, where she has spent the majority of her career to date.
The switch sees the 24-year-old reunite with new Roma Women coach Alessandro Spugna, who led the Azzurre last season.
"I think that this move to Roma comes at the right moment in my career," Di Guglielmo said.
"I feel ready to make that step forward in my career, and I'm delighted to have the chance to do it with this club."
With Empoli the full-back not only helped the side reach Serie A, but eventually took on the captain's armband as the side finished seventh in Serie A last season.
"Lucia is a player with a lot of experience already under her belt, despite her young age," said the club's Head of Women's Football, Betty Bavagnoli.
"Her character has already seen her wear the captain's armband and earn international honours.
"We really wanted her here, not just for her undoubted quality on the pitch but also for her qualities as a person."
Di Guglielmo is the club's second signing of the summer, following the arrival of Benedetta Glionna.
Welcome to the club, Lucia!
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