The hot seat at the Team of Choice has been vacant for the past month after the club parted ways with Clive Barker, who had earlier taken over from Mandla Ncikazi.
Ncikazi has previously been the assistant coach to Steve Komphela, the man who guided United to a top-eight finish last season before linking up with Kaizer Chiefs.
With the KZN outfit currently occupying last position in the league standings after only one win this season, Middendorp's task will be to help them climb to safety in the 18 games still remaining this term.
The German joins Maritzburg after parting ways with Free State Stars last week. Before that, he was at Bloemfontein Celtic and Chippa United. He was head coach of Maritzburg between 2009 - 2011, and again in 2012/13.