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 new virus - one of the highest tallies outside China.
Meanwhile, Formula 1's Chinese Grand Prix in Shanghai has been postponed due to the outbreak.
Motorsport governing body FIA said the measure had been taken "in order to ensure the health and safety of the travelling staff, championship participants and fans". It said it hoped to find an alternative date later in the year "should the situation improve".
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 was confirmed on Wednesday.
"As a precautionary measure, all employees on the affected floor vacated the premises to work from home," the firm said in a statement.
DBS said "during this difficult time, the bank will be providing this employee and his family with every support and guidance".
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 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.
More about: Singapore