The woman, 84, was pushed to the ground at Saltwells Nature Reserve in Netherton, after being approached by a man who then stole her handbag.
Police are hunting a suspect who is described as white and was dressed all in black and wearing a face mask.
The incident has left the woman "very shaken", say officers, who are appealing for information.
The robbery took place on Tuesday at around 4pm while the woman was walking with a friend.
Detective Sergeant Leighton Shingles, from force's Crime Investigation Department, said: "The elderly woman remains in hospital with serious injuries and has been left very shaken by the incident.
"I would urge anyone who witnessed the attack or has any information no matter how small, to get in touch."
A spokesman for West Midlands Ambulance Service added: "We were called to reports that an elderly woman had been hurt when a man stole her handbag. An ambulance was sent to Saltwells Nature Reserve, Copse Road, Netherton after a 999 call at 4.29pm.
"The woman was assessed and treated at the scene before being taken to Russells Hall Hospital for further assessment. A second woman who was with the patient was shaken but unhurt by the incident."
Anyone with information is asked to contact police quoting crime reference number 20DY/170947F/20.