Bangkok Metropolitan Administration (BMA)'s Erawan Medical Centre reported that 24 persons were injured in the bomb attack and three later died at hospital.
The latest casualty was a 6-year-old girl identified as Patcharakorn Yos-ubon. Doctors at Ramathibodi Hospital said she died from injuries to her brain and liver.
Her younger brother Korawich, a 4-year-old boy, was earlier pronounced dead at Ramathibodi Hospital after receiving severe brain injuries.
Another victim was 59-year-old woman named Tipapan Suwanmanee, their aunt, who died at Phyathai 1 Hospital.
There were 11 victims who were treated at various hospitals - Rajavithi, Chulalongkorn, Ramathibodi and Phramongkutklao.
The bombing occurred at about 4pm on Sunday. Police said it was believed that the explosion was a 40mm round shot from an M79 grenade launcher from high- rise building outside the protest area of the People's Democratic Reform Committee (PDRC).
Police said the M79 launcher could shoot from 300-400 metres away and this attack was aimed for casualty as the target was the crowded area.
The grenade fell near a 3-wheel tuk tuk- taxi delivering passengers near protest souvenir vendor stalls.
Police were investigating the incident, but surveillance camera footage revealed no clues to investigators.