The three-story house on Pham Ngoc Thach Street belonged to Doan Thi Manh, 74. The fire began on the first floor, then spread to the other floors, Hanoi police said.
Firefighters were dispatched to the scene at 1:18 a.m., about five minutes after someone informed about the fire. At that time, one of Manh's sons and a 12-year-old grandchild had managed to escape to a house next door and were rescued.
The flames were extinguished at around 1:23 a.m. Police found a one-year-old boy, Manh's grandson, on the second floor and took him to the hospital, but he later died.
The four other casualties were Manh, her son and daughter-in-law, and her 10-year-old grandson.
An investigation is ongoing to figure out the cause of the fire.