The Western Trust had to put additional measures in place after the situation arose at Altnagelvin Area Hospital in Derry at around 2pm.
South West Acute Hospital in Enniskillen was put on standby to take extra patients if necessary.
Ambulance crews were told not to bring non-urgent patients to Altnagelvin to ease the burden on staff.
The additional measures were lifted at around 4pm.
In a statement on social media, the Western Trust said: “Our ED (Emergency Department) at Altnagelvin is busy at present. Please only attend if your condition is urgent or life-threatening.
“The current situation has required the Trust to activate additional measures to help manage the pressure in ED.
“Please consider other healthcare options available to you.”