The 22-year-old has joined Nice on a four-year contract and strengthens a team that still has some doubt surrounding the future of number one David Ospina.
Delle is a France youth international and made 10 starts in Ligue 2 last season for Metz.
"Joris has many qualities," said coach Claude Puel. "He is a goalkeeper of good stature who has presence in the air."
Meanwhile, Nice have also confirmed that Mali international Drissa Diakite has left the club on a free transfer.
Diakite joined the club in February 2006 and made 156 appearances in Ligue 1.