Richard Hinds has been found guilty of the murder of Irish student Nicola Furlong in Tokyo last May.
He was given a sentence of a minimum of five years and a maximum of ten years with labour.
Chief judge Masaharu Ashizawa said this was the maximum sentence possible because Hinds was a minor under Japanese law.
He said Hinds' testimony was "not credible" and had "tainted the honour of his victim".
Judge Ashizawa said the court concluded he had strangled Ms Furlong using either a towel or a tank top, both of which had traces of her DNA.
He said the attack was "atrocious and vicious in nature".
The 21-year-old Wexford woman was found dead in the Keio Plaza hotel in Tokyo's Shinjuku district on 24 May last year.
Hinds, 19, from Tennessee in the US, had denied her murder.
He showed no emotion as the judge delivered the verdict. He briefly said goodbye to his family before he was led away.
Ms Furlong's family have expressed anger at the sentence handed down to Hinds.
Speaking outside the court, Ms Furlong's mother Angela said: "Nicola's life was worth more than that. It really, really was. It's good that they've cleared her name.
"Nicola had done nothing wrong. It was all him and his lies that brought us here. We still don't know the truth, what happened in that room, but we know that Nicola did nothing wrong. We knew that coming out anyway."
Ms Furlong's sister Andrea said she was "absolutely disgusted" at the sentence.
"I'm so angry and I'm so hurt. We had so much faith in the Japanese doing justice for us and I don't feel we got it. So I'm disgusted," she said.
Reacting to the sentence, her father Andrew said: "We said at the start it was going to be down to the Japanese.
"When we found out he was going to be tried as a minor, we knew what he was going to get ... We hope it's more than five and up to ten."
Family angry at portrayal of Ms Furlong
Mr Furlong reiterated the family's anger at how his daughter was portrayed by the defence.
He said: "The way they tried to paint Nicola, there's not a hope in hell was she like that. All her friends knew her.
"I hope everyone that didn't know her realises that she wasn't a bit like they were trying to make out.
"We got her name cleared, and that's what I came out [for] and hopefully to get more of a sentence, but that didn't happen. We went with the Japanese. It's their country. I'll take it.
"I don't want to ever hear his name mentioned again, so whatever happens from now on, I've no interest in him whatsoever."
Mrs Furlong said her daughter's honour had been restored by the verdict.
She said: "We know what type of little girl Nicola is. She always will be a very special little girl to us.
"She's not what that person [the defence] tried to make her out to be and thankfully the whole nation, the whole world can see that now. Nicola's a good girl."
Mrs Furlong said the family would go home and try to move on. Andrea Furlong said she would never return to Japan.
Her father said it would not be easy to settle back. He said there would be emptiness, but they could now start their grieving and hopefully Ms Furlong could rest in peace.
Ms Furlong had been in Japan as an exchange student from Dublin City University.
She had been studying at the Takasaki City University of Economics in Takasaki, Gunma Prefecture, northwest of Tokyo.