Keith Baker, 60, and his wife Caroline, 54, had previously denied the charges.
Baker, whose address was given as Maghaberry Prison, admitted the rape and assault of the woman over an eight-year period.
His wife admitted she helped him and indecently assaulted the woman herself.
Two charges of false imprisonment were left on the books.
The court had previously heard that the couple kept her prisoner in squalid conditions in what was described as a "house of horrors".
The couple will be sentenced on 2 December.
Caroline Baker was released on continuous bail while her husband was remanded back into custody.
A husband and wife from County Armagh have admitted a number of sex offences, including the rape and assault of a woman with severe learning difficulties.