The 19-year-old woman was taken to hospital and remained there overnight.
The boy is suspected of attempted rape. The social services and the boy's parents were in attendance when the boy was interviewed by police on Thursday evening.
The attack occurred at around 7pm. The woman and the boy were on the same bus and the boy is alleged to have been sexually harassing the woman prior to the attack, according to local newspaper Borås Tidning.
The boy got off the bus at the next bus stop after the woman and walked back to continue his harassment of her.
He then proceeded to punch her in the face and grab her, after which he dragged her trousers to her knees and stated that he wanted to have sex.
It was at this point that a family in the vicinity heard the woman's cries and came to her rescue.
"It was a very timely intervention," Bo Dahleman at Borås police told the newspaper.
The attack is the fourth in the area in recent days. The newspaper reports on Friday morning that police suspect another, 13-year-old, boy to be behind the other attacks.
Both boys remain in custody and are being held at a youth detention centre while social services decide what action to take.
A press spokesperson at Borås hospital confirmed on Thursday evening that the woman's injuries are considered to be relatively serious.