He has joined Paris FC, the club he served on loan in the second half of last season, on a permanent transfer.
Albion have signed three central defenders in recent weeks.
Sporting director Mike Cave said: “Diego made a good impact with Paris during his loan there last season and it was a transfer both he and Paris FC were keen to pursue.
“With growing competition for places, this moved suits all parties and allows Diego to make the next step in his career. We wish him all the best.”
Coppola impressed with his efforts in training for the Seagulls and on the rare occasions he got on the pitch.
But he was frustrated at his lack of playing time, having been a central figure at the back for his previous club Hellas Verona.
Coppola reported back for tests and training with Albion at their Lancing base last week.
He shone in Paris under the guidance of head coach Antoine Kombouare, himself an uncompromising centre-back in his playing days, as they climbed the Ligue Un table during the second half of last season.
Kombouare has since been replaced in command by former Albion full-back Liam Rosenior.