Dedi III (German: Dedo), nicknamed the Fat (c. 1130 – 16 August 1190), a member of the House of Wettin, was Margrave of Lusatia from 1185 until his death.

Dedi was a younger son of the Wettin margrave Conrad I of Meissen and his wife Luitgard of Ravenstein.

When Margrave Conrad retired in 1156, the extended Wettin estates were divided and Dedi formally received the County of Groitzsch and the Lordship of Rochlitz, with jurisdiction over the Bishopric of Naumburg.

Like his elder brother Margrave Otto II of Meissen, he encouraged the settlement of ethnic Germans in his territory.

In the succession dispute in Meissen upon the death of Margrave Otto II in 1190, Dedi and his sons sided with their cousin Albert the Proud.