Police, paramedics, firefighters and helicopters searched the area off Highbridge Road on Tuesday evening after reports of multiple people being in the water.
The woman was pulled from the water before fire crews were called at around 5.45pm, said watch commander Danny Hinds, from Brierley Hill Fire Station. She was taken to hospital in a serious condition,.
Crews from the fire service's Technical Rescue Unit continued to search the reservoir but nobody was else was found and they left the scene at 7.15pm. The search is not continuing this morning and the woman is believed to have been the only person in the water.
Watch Commander Hinds told the Express & Star: "We received reports there were further casualties in the water.
"We had all of our services searching the area for a possible sighting but no further casualties have been located."
Claire Brown, spokeswoman for West Midlands Ambulance Service, said: "West Midlands Ambulance Service can confirm that it was called by the police to reports of a patient in the reservoir on Highbridge Road, Netherton at 4.39pm yesterday.
"Two ambulances, a paramedic officer and a critical care car attended the scene.
"Crews arrived to find a woman who had been rescued from the water and was in a critical condition.
"She was given advanced life support by the team of ambulance staff on scene before being conveyed on blue lights to Russells Hall Hospital for further emergency care."
It is not known how the woman came to be in the reservoir.
A spokesman for West Midlands Police said: "We currently have officers at Netherton Reservoir, Dudley, after a woman was spotted in the water.
"She has been taken to hospital in a serious condition. Police continue to search the area."
Three fire crews, and a total of 15 firefighters were standing by at the request of the police, West Midlands Fire Service said.
The incident happened one week after a man was found dead in the same reservoir.
Police divers found the man's body last Tuesday after a search which started on Monday afternoon.
His death is not being treated as suspicious, a police spokesman said.