The minister, who was inside meeting the public on an open day, escaped unhurt.
Devananda's Eelam People's Democratic Party (EPDP) office is on Park Road in the Narahenpita area of Colombo city limits.
Devananda, a former Tamil militant, has survived several attempt on his life by the Tigers.
The bomb blast, which occurred at 08.05 a.m. was carried out by a handicapped female suicide carde of the Tigers, military officials said.
The wounded, two in serious condition, were brought to the city's General Hospital.
"The three people who are injured are security people for the minister," hospital spokeswoman Pushpa Soysa told AFP.
The military said they were still checking on casualties and whether the bomber was dead.
The blast came a day after Tamil Tiger rebel supremo Velupi