Jamna, who is said to be mother of Sai's son, had evaded questioning by police since he was arrested on December 4 in connection with the rape case.
"Narayan Sai's female disciple Jamna, with whom Sai had fathered a child, surrendered today before Surat police. We will arrest her and produce her before court tomorrow," DCP (Crime) Shobha Bhutada told PTI.
Police said Jamna had been named in the original FIR and her interrogation will strengthen the case against Sai.
Jamna had informed police that she was in an ashram in Madhya Pradesh.
Sai was apprehended from Haryana-Delhi border after evading arrest for nearly two months in the sexual assault case lodged against him by an inmate of his Surat-based ashram.
Sai himself had admitted to police on December 11 that he had had physical relationship with eight of his female disciples.
Two sisters had lodged complaints against Sai and his father Asaram of rape, sexual assault, illegal confinement and other charges.
Surat police had booked 40-year-old Sai under various sections including rape, unnatural sex, molestation, wrongful confinement, unlawful assembly, rioting armed with deadly weapon, criminal intimidation and criminal conspiracy of the IPC.
In her complaint, the younger of the two sisters had accused Sai of repeated sexual assault between 2002 and 2005 when she was living at the Surat ashram.
The elder sister, in her complaint against Asaram, had accused him of repeated sexual assaults between 1997 and 2006 during the time she was living at his ashram on the outskirts of Ahmedabad.
The complaint against Asaram, which was initially lodged at Surat's Jehangirpura police station, was transferred to city's Chandkheda police station as the alleged incident took place here.
Asaram (72) is currently lodged in a Jodhpur jail after being arrested in September for allegedly sexually assaulting a minor girl at his Jodhpur ashram.
A chargesheet was filed against Asaram in a court in Gandhinagar last week accusing him of unnatural sex, rape and illegal confinement.