Mark Angell lashed out at the boy, 15, when his daughter phoned him to say she was being abused by youths in Barnstaple, Devon, and was hiding out in a shop.
Her father rushed to her aid and struck a boy who he thought had been involved, but the actual perpetrators had already fled the scene.
Angell, a 38-year-old tattooist punched the 'young looking' 15-year-old boy in the face, leaving him with a fractured cheekbone.
Magistrates in North Devon earlier found Angell guilty of actual bodily harm and transferred his case to Exeter for sentence. Angell had denied the charges.
Jailing Angell for nine months, Judge Francis Gilbert, QC, told him he had made 'no attempt' to find out if the boy he'd struck had been the person abusing his daughter.
"This offence was against a small, slim, young looking 15-year-old who you punched, causing him a black eye and a displaced fracture of the eye socket which must have been very painful and taken several weeks to recover from," the judge said.
"You told police you thought he had been hassling your daughter but you made no attempt to find out if it was the correct person before you hit him. You claimed to be acting in self defence but for a man of your age, with your record, and bearing in mind he was a just a schoolboy, that is nonsense.
"Another young person who saw what you did said they were shocked and disgusted that an adult like you would assault a young boy."
The court had heard that Angell had a record of violence which included three previous offences of battery and one of threatening behaviour.
Janice Eagles, prosecuting, said the attack was serious because of the discrepancy in age between Angell and the boy.
Mr Nigel Wraith, defending, said his client had lashed out because he thought the boy was going to hit him.
"He had gone from his home to look after his daughter. He was upset and angry about what had been happening to her and he acted out of misguided loyalty," he said.
"There was only one punch and he wishes the young man no ill will and feels remorse for what he has done. My client has been diagnosed with post traumatic stress disorder which results from an extremely severe childhood."
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