After just three days of testimony in the trial of Brian Stewart, 32, in St Charles, Missouri, the jury delivered its verdict late on Saturday. Sentencing was set for 8 January.
Stewart, who listened impassively to the verdict, is expected to appeal.
Prosecutors told the court that Stewart, who worked in a hospital as a phlebotomist - someone who draws blood - stole the HIV-infected blood from his workplace and then injected it into his son while the boy was in hospital for treatment for asthma.
The jury heard how Stewart committed the crime during 1992, when the boy was 11 months old, because he did not want to pay for child support. Stewart was already separated from the boy's mother, identified only as "Jennifer" to protect the identity of the child.
Jennifer said her son now suffers from full-blown Aids, which was diagnosed four years after the crime. He is kept alive by a combination of potent drugs, has lost most of his hearing, is fed through a tube in his stomach, and has come close to death several times. On the stand last week, she said Stewart had told her that the boy would die. "He said, `You won't need to look me up for child support anyway because your child is not going to live very long'," she testified.
A county sheriff's detective testified during that Stewart, who worked in the St Louis area, had a history of drawing more blood from patients than was necessary, had sometimes "lost" samples and had extensive contact with Aids patients in his work.
Joe Murphy, lawyer for the defence, said the prosecution had built a case out of circumstantial evidence and had failed to offer any proof. "A tragedy is not a crime and theories are not facts," he told reporters. "Mom made an allegation and everyone ran with it."
Mr Murphy had tried to show that the boy could have contracted the virus from other sources, including intravenous drug abusers and convicted child molesters who shared the mother's home from time to time.
But prosecutors said all 23 adults with whom the boy had come into contact tested HIV negative, and there was no evidence that the boy had been sexually abused.
The boy's mother sobbed as the sentence was read out. "My son has been robbed of a normal childhood and given an unjust sentence of his own," she told reporters after the verdict.