The third arm protrudes from the center of the little girl's back, near her spinal cord, The Express Tribune reports.
Dr. Ghulam Shabbir of Nishtar Hospital in Multan says the baby, born Sunday, has a rare genetic disorder called polymelia, which causes a person to be born with extra limbs.
Shabbir told The Express Tribune that experts are consulting to determine how difficult it will be to remove the third arm in surgery. It will be the first operation of its kind at the hospital.
The baby was transferred to Nishtar on Wednesday, where her father says she is undergoing treatment prior to the surgery, according to The Express Tribune.
Earlier this month, also in Pakistan, a baby was born with six legs. Last Thursday, doctors successfully removed the extra limbs.
Polymelia affects fewer than one in a million babies, Jamal Raza, head of the country's National Institute of Child Health, told Pakistani website Dawn.com.