Emergency services were called to the park near Walsall town centre at around 8.30am on Wednesday after receiving reports that a woman had been spotted in the lake.
The woman, believed to be in her 30s, was pulled from the water by fire service rescue swimmers being taken to hospital in a critical condition.
A West Midlands Fire Service spokesman said: “Two fire engines from Woodgate Valley and Walsall fire stations attended the scene with specialist equipment from Willenhall and our technical rescue team.
“Working with West Midlands police and ambulance services, and with the aid of an ambulance service drone, two of our rescue swimmers retrieved her from the water.
“She was taken to hospital by ambulance.”
Witnesses reported seeing crews by the bandstand at the Arboretum.
A West Midlands Ambulance Service’s specialist Hazardous Area Response Team was sent to the scene along with an ambulance, two paramedic officers and a critical care car.
A spokeswoman for the ambulance service said: “Upon arrival, we found one patient who was a woman in a critical condition, she received treatment on scene and was conveyed to Queen Elizabeth Hospital Birmingham for further treatment.”