The Rio de Janeiro State Health Department said in a statement that the woman's husband was cleaning his spear gun when it went off, firing a harpoon that hit her cervical spine.
Elisangela Borborema Rosa was rushed to the hospital and underwent emergency surgery after Monday's incident in the coastal city of Arraial do Cabo.
The statement quotes neurosurgeon Allan da Costa as saying that the harpoon came within 1 centimeter (less than half an inch) of killing the woman. He said he expects a full recovery.
A police officer in Arrial do Cabo said by telephone that officials are looking into the case.
"Everything indicates it was an accident, but we are investigating. We don't think the husband tried to kill her," said the officer, who cited department policy in declining to let her name be used.
"But once she fully recovers we will be able to question her and get a clearer picture of what happened."