Five fire engines went to the scene at Sadlers Wells Court in East Kilbride after the alarm was raised at around 10.20pm on Thursday.
The body of a 65-year-old woman was recovered from a property in the block.
Crews were still at the scene in the South Lanarkshire town on Friday morning.
Police also went to the incident on Thursday evening.
A police spokesman said: "The fire was extinguished by the Scottish Fire and Rescue Service (SFRS) and the body of a 65-year-old woman was recovered from the property.
"A post mortem will take place in due course to establish the cause of death.
"A joint investigation will be carried out with the SFRS to establish the cause of the fire."