Middendorp takes up the role after yesterday resigning from fellow Premier Soccer League outfit Maritzburg United.
The position became vacant when Clinton Larsen quit Phunya Sele Sele on Monday.
"I am excited to take on this challenge knowing the huge support that this club has and the existing resources that they have invested," Middendorp said in a statement released by the German's agent, Steve Kapeluschnik.
"I am looking forward to coaching the players and I am passionate about helping the club fulfill its potential."
Middendorp, whose contract runs until 2016, is expected to be in the stands when Celtic contest the annual Macufe Cup with one of his former clubs Kaizer Chiefs at Free State Stadium on Sunday.