The city's flagship sports centre was closed down for the afternoon after suffering a power cut which affected hundreds of nearby properties.
Bosses at the Stadium Park venue said the venue would be closed until further notice as a series of buildings in the area were hit by the issue.
However, it reopened at around 4.30pm after power was restored.
Northern Powergrid reported the issues had been caused by an underground cable supplying the area.
A spokesman said 611 customers in the area suffered a cut from 2.50pm, with power to all but one restored by 3.42pm.
The company said that one remaining customer is expected to have their supply back by 6.30pm.
It has confirmed the issue is unrelated to the failure which hit Sunderland's home game at the neighbouring Stadium of Light on Saturday.
Steve Dougal, Everyone Active’s general manager at the Sunderland Aquatic Centre, said: “As a safety precaution, we took the measure to temporarily close the centre until power was
“Engineers carried out work to a substation in Southwick to rectify the problem, and the power was back on approximately two hours later.
"The centre is now open as normal.
“We apologise for any inconvenience to customers during the outage.”
The centre is home to the city's Olympic-sized pool, gym and soft play area, with classes and functions held in its studios.