Annette Henderson was a learning support worker at Maes Ebbw School when the incident happened in January 2016.
The Education Workforce Council hearing in Cardiff found allegations of unacceptable professional conduct against her proven.
She was issued with a reprimand but has not been struck off from teaching.
The disciplinary panel heard the girl - who was known as "Pupil A" and has learning difficulties - was tied to the chair with the sleeves of a hooded top in a double knot after allegedly hitting another pupil.
Mrs Henderson, from Cwmbran, Torfaen, then left the classroom leaving another member of staff to untie her.
She was reported to the deputy head teacher and an investigation was launched at the school.
Mrs Henderson, who had been working at the school for 16 years, admitted tying up the pupil but denied using a double knot or that her behaviour amounted to unacceptable professional conduct.
She told the hearing: "I would never do anything to harm any child.
"I know it looks bad but it wasn't done maliciously, it wasn't done as a restraint but from a safety point of view."
Mrs Henderson was found to have breached school rules on child protection and restraining pupils at the hearing.
A teaching assistant who tied a six-year-old girl to a chair at a Newport special needs school will be allowed to return to the classroom.