The young striker - son of legendary Moroka Swallows striker Les Grobler - joined Matsatsantsa a Pitori from Ajax Cape Town.
Before he joined Ajax last year, Grobler junior had a stint in Turkey where he played for Gotzepe AS after impressing them while with Platinum Stars.
Grobler will have to continue working hard to be in the starting line-up of United where he will be competing for the same position as Kermit Erasmus, Ashley Hartog, Nathan Paulse, Sibusiso Zuma, Atusaye Nyondo, Prince Nxumalo and Thabiso Nkoana.
The Johannesburg-born Grobler won two caps for Bafana Bafana. He scored on his international debut against Zimbabwe two years ago.
Confirming his arrival in Randburg, United's communications manager David Skhosana said: "He will definitely add value, we are happy he joins us when we are busy at training because of the Afcon break".