All 300 had been working on the same floor, the 43rd, and were sent home on Wednesday.
Singapore had previously reported 47 cases of the coronavirus – one of the highest tallies outside China.
The overall number of infections is more than 44,000 on mainland China, with cases in more than 20 countries.
The DBS employee was tested on Tuesday and his infection (coronavirus) was confirmed on Wednesday.
DBS said “during this difficult time, the bank will be providing this employee and his family with every support and guidance”.
Building management staff have been testing people’s temperatures in Singapore’s financial district
Contact tracing is being done to verify who had been exposed to the affected employee. The common areas of the building in the Marina Bay Financial Centre – such as lifts and toilets – are being deep cleaned and disinfected.
Care packs with thermometers, masks and hand sanitisers were given to the evacuated employees and the bank said a medical helpline was in place.
Singapore raised its “Disease Outbreak Response System Condition” to orange last week – which means the disease corornavirus is severe and spreads easily from person to person.
As a consequence, all companies and many public spaces have started scanning employees and visitors for temperature.
The Covid-19, as the disease is officially called, has killed more than 1,100 people in China, where it emerged.
On Wednesday though, the country reported its lowest number of new cases in almost two weeks, with 2,015 new confirmed cases on Tuesday.
In its latest measure to try to halt the spread, China has said it will stagger the return of children to school. Several provinces have closed schools until the end of February.
In Japan, the number of infections on a cruise ship quarantined off Yokohama has risen by 39.
With 174 confirmed cases, the Diamond Princess is the largest single cluster of the virus outside China.
From a total of more than 3,700 passengers and crew on board, almost 500 have been tested so far, but there are plans to step up testing in the next days.
While those infected with coronavirus are brought on land and taken into medical care, the remaining passengers have to stay in quarantine on the ship.