The emergency department will also not be taking ambulance admissions overnight, between 18:00 to 08:00 GMT.
It is understood the decision was taken after a meeting of clinicians.
In a statement the Belfast Trust said it operates its two EDs as one emergency service.
The Belfast Health Trust said: "The two emergency departments work together and a temporary ambulance divert is an appropriate mitigating action and ensures those patients who arrive by ambulance and who are often in greatest need are seen in a timely way.
"A temporary arrangement to divert out of hours ambulances from the Mater Hospital's emergency department to the Royal Victoria Hospital's ED is in place."
The trust added that the arrangement would be kept under continual review.
Children attending the Mater Hospital's emergency department (ED) in north Belfast this weekend will be redirected to the Royal Belfast Hospital for Sick Children.