Cilliers, a former flyhalf who played one Test for the Boks in 1996, started working as the Bulls' kicking coach in 2004.
However, Cilliers has only been used on a consultancy basis by the Bulls in recent times, and there were rumours he may move back down to the Cape, where he played his rugby in the 1990s.
According to the Beeld website, Cilliers will join WP as soon as next month.
This is the third step in the right direction as the WP Rugby Union attempts to improve its coaching structures. Gert Smal was appointed Director of Rugby in March after the Stormers' season took a turn for the worse. Smal also recently appointed Henning Gericke as sports psychologist.
Since these appointments, the Stormers have showed marked improvement, and won four out of five games last month.