The fire quickly killed 13 people, most aged between 18 and 25, after it broke out during a birthday party on August 6 in the Au Cuba Libre bar in Rouen.
Six others remain injured as a result of the blaze.
"The family of Karima Ababsa is sad to announce the death of the young woman this afternoon after a month of suffering," lawyer Gerard Chemla said in a statement.
The 25-year-old's condition had deteriorated in recent days and a decision was made to "stop her treatment" in order to put an end to her suffering, the lawyer added.
The increased toll from the fire tragedy was confirmed by the Rouen prosecutor's office.
An investigation will focus on whether building safety rules were respected at the bar, particularly in relation to fire exits and flammable materials.
It is the highest death toll in a French night spot fire since November 1970, when a blaze at a disco in the east of the country claimed the lives of 146 people, most in their early 20s.
