Three of the dead are reported to be minors. One of them is said to be a school-age child and another a toddler.
"I can confirm that a total of 25 people were shot," said police spokesperson Athlenda Mathe, adding that 14 people have been taken to hospital.
The hostel is located in Saulsville, just under two miles west of Pretoria city center.
The shooting took place at 4:30 a.m. on Saturday. The motive for the shooting is unknown and no one has been arrested.
The shooting is the latest in a series of mass shootings that have shocked crime-ridden South Africa, with police saying around 63 people were killed every day between April and September.
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