Desderia Mbwelwa, 57, has been sentenced to 29 years in prison at a court on Friday but details of the verdict have not been widely reported until now.
On the day of the attack, the woman found the victim herding cattle at a village in southern district of Iringa and asked where his friends were.
She is then said to have sexually assaulted him under a tree after he said that his friends were not around.
The case had five witnesses including a doctor who examined the boy and confirmed he had injuries and had contracted sexual infections due to contamination in the genital area.
Mbwelwa is said to have defended herself by saying she is an adult with children and grandchildren who depend on her.
Her lawyer, Frank Mwela, said that he plans to appeal against the verdict because his client was not tested to confirm if she indeed had sexually transmitted diseases.
“My client was not tested positive for those diseases, which both my client and her witness have confirmed they do not have because one of the witnesses is her husband,” he said.