The type locality for Robertsite is the Tip Top mine, Custer County, South Dakota, US.

Robertsite occurs at the Tip Top Mine as minute crystal aggregates and crusts found on quartz associated with triphylite.

[3] It is associated with rockbridgeite, ferrisicklerite, leucophosphite, jahnsite, montgomeryite, collinsite and hureaulite in the type locality.

[3] In the Omo Valley of Ethiopia it occurs with mitridatite associated with fossil fish in Pliocene/Pleistocene lake sediments.

[5] Recently, in an exploration conducted by the Italian La Venta Geographical Association, confirmed the existence of Robertsite in the Puerto Princesa Subterranean River National Park, located about 50 kilometres (31 mi) north of the city center of Puerto Princesa, Palawan, Philippines.