People with non-life-threatening conditions are being asked to make an appointment ahead of attending the accident and emergency unit at the University Hospital of Wales.
Cardiff and Vale University Health Board is introducing a "phone first" triage system for the emergency unit in Cardiff from August 5.
The new system, called CAV 24/7, has been introduced as a response to the coronavirus pandemic so the health board can ensure that the emergency department does not become overcrowded.
Patients requiring urgent care will be assessed and signposted to the "most appropriate medical help", the health board said.
Those suffering life-threatening emergencies - such as symptoms of a stroke, loss of breathing or a suspected heart attack - should still call 999.
Traditionally, patients could access the emergency department by walking into their local Emergency Unit (EU) or A&E.
The health board said that returning to how patients accessed the A&E unit before the coronavirus pandemic was not "deemed safe for our patients or staff".
Consultant Dr Katja Empson said: "By introducing this system, we believe it will help keep our patients and staff safe as it helps reduce overcrowding in the waiting room, allows us to socially distance, and patients can wait in the comfort of their own homes for their scheduled time slot.
"It's important to emphasise that this will not replace 999 calls; if you have an emergency that is life-threatening such as symptoms of a stroke, loss of breathing or a suspected heart attack then you must still call 999.
"This process will not change."
A similar system is already in operation in Denmark, where all but the sickest patients must call ahead to make an appointment before attending an A&E unit.
When the plans for Cardiff's A&E were announced in July, Finance Minister Rebecca Evans urged people to seek medical help where required and said the NHS is "open for patients".
Ms Evans said people should not feel "worried or concerned" about phoning for an ambulance or approaching A&E if they require urgent medical attention.
"I wouldn't want anybody to be under any impression that A&E was not the place that they should go if they need to, or that it wouldn't be a safe place for them to go," she added.