Police are treating the death as "unexplained" and have launched a joint investigation with the Scottish Fire and Rescue Service (SFRS).
Fire crews were called to the scene in Glenapp Place in Kilwinning, North Ayrshire, in response to a 999 call to the fire service soon after 2am today.
When they arrived they found a "well developed" fire and the building filled with toxic smoke.
They discovered the woman, who was confirmed to have died at the scene.
The SFRS teams rescued a man who was passed to the care of paramedics before being taken to hospital.
Firefighters also treated a second woman from an adjacent property for minor smoke inhalation.
Station manager Scott Cunningham, the incident commander, said: "The thoughts of all the emergency responders will be with the family and friends of the woman who lost her life as well as with the man and woman who were injured.
"Our crews arrived at this incident within minutes of the 999 call but they were already faced by a well-developed fire producing large amounts of toxic smoke.
"We immediately committed teams to the building and a total of eight firefighters in breathing apparatus were operating inside the property, conducting both search and rescue and firefighting duties.
"Their skill, courage and professionalism ensured a man was rescued from the smoke-filled building and taken to hospital, however tragically a woman found inside the property was confirmed to have died at the scene."
Police appealed for information about the blaze.
Detective Inspector Stevie Wallace at Saltcoats CID said: "A post-mortem examination will be carried out to establish the cause of death which is currently being treated as unexplained.
"The woman has still to be formally identified and we are therefore unable to provide further personal details at this time. I would appeal to anyone who was in the area of Glenapp Place, Kilwinning, around 2am on Tuesday morning to get in touch."