The explosion took place in Pettah, a market area close to Fort, a heavily guarded district which is home to a number of government offices and the presidential palace, during the lunchtime rush hour.
It was the sixth blast in the capital since August 30 when an explosion in Pettah wounded 45 people, and it came just a day after Sri Lanka's military said a suspected suicide bomber had killed at least one person and injured eight others in the northern town of Vavuniya.
The army has been carrying a major offensive against the Tamil Tigers in the north of the island. The military has said it expects an increase in attempted attacks as the separatists' conventional fighting strength is reduced by the fighting.
The military said air force helicopters attacked Tamil Tiger bunker lines on Monday in support of ground troops in the Kilinochchi district, near the separatist's de facto capital.
The LTTE has been fighting for an independent homeland in the north and east of the country for more than two decades.