A benefactress is a female benefactor.
The feminine form of benefactor is benefactress.
That would be a benefactress.
benefactress
Rosamond Oliver
benefactor, benefactress, contributor, donator, giver
There is no grammatical gender in the English noun. The word benefactress may be used for a female benefactor, but it is not necessary except to specify the person's gender, which of course is a form of sexism.
The benefactress of Lowood School in Jane Eyre is Mr. Brocklehurst's wealthy aunt, Miss Brocklehurst. She provides financial support to the school and has influence over its operations.
J. J. Colter has written: 'A gentle benefactress'
Benefactor is technically gender neutral, but benefactress is a specifically feminine form of the word.
Masculine
It is masculine.