Police say the woman's son was inside the house at the time she was attacked.
Officers say he was also hit with the hammer as he ran to his mother's aid. He fought with the offender before he ran off.
Police have issued the following description of the man:
- 25 to 29 years old
- Medium build
- 5ft 8ins - 5ft 9ins
- White with some stubble
- Wearing a light baseball cap, grey long-sleeved top and jeans
Anyone who may recognise the description or have any information are asked to contact Detective Constable Andy Jones via the 24-hour non-emergency number 101, or Crimestoppers on 0800 555 111.
Investigating officer Det Con Andy Jones from Avon & Somerset Police called the attack on the 81-year-old woman 'horrific'.
– Det Con Andy Jones, Avon and Somerset Police
We believe the offender may have followed the woman from the Post Office back to her home.
She suffered nasty head injuries and although she is recovering in hospital this attack could easily have had more serious or even fatal consequences.
I am asking the public to think about whether they saw anyone hanging around the Post Office in Sea Mills around lunchtime, or who may have seen a man following an elderly lady in the Sea Mills area.
If you recognise the description of the offender, or have any information which could help us catch the person responsible, please call us as soon as possible.
An 81-year-old woman has been attacked with a hammer during a robbery in Bristol.
The woman had returned to her home in the Sea Mills area of the city yesterday afternoon after collecting her pension from the Post Office in Shirehampton Road.
As she unlocked the door, the pensioner was attacked from behind by a man who hit her around the head with a hammer several times.