Frenchman who joined Dover in July 2018 after leaving Barrow AFC.
The imposing (6ft 5ins) 28-year-old central defender and former Paris Saint-Germain junior spent two years with the Bluebirds after signing from National League South side Hampton & Richmond Borough.
In his time in Cumbria, Diarra made 88 appearances in the National League and FA Cup, scoring eight goals, including one against Whites in the final game of the 2016/17 season at Holker Street.
The Parisian becomes manager Chris Kinnear’s fourth new summer signing following the arrival of his former Barrow team-mate, midfielder Bedsente Gomis, defender Kevin Lokko and striker Inih Effiong.
Diarra arrived in the UK in 2011 and played for St Albans City before moving to Hemel Hempstead and then winning promotion from the Ryman League with Hampton & Richmond Borough.