The teenager of Turkish origin was found guilty of belonging to a "terrorist" organisation and given an additional suspended jail sentence of 16 months by a court in his hometown of Sankt-Poelten.
According to the charge sheet, the teenager, who emigrated from Turkey in 2007, wanted to carry out the attack before travelling to join "holy war" in Syria alongside the Islamic State (IS) group.
Police had said at the time of his arrest in October 2014 that the boy made "concrete enquiries about buying ingredients" for a bomb and "planned to explode the devices in public places, such as the Vienna Westbahnhof," a major train station.
Placed in a special school for troubled children where "prospects for professional development are almost inexistent", the boy had "looked for recognition, to belong to something," Mayer told the court.
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"Imagine the power of propaganda that says to young people who feel they are living an empty existence: 'You can do something good, and get money and women'," he said.
It was his own family who alerted authorities to his growing radicalisation.
The baby-faced teenager appeared in court in a grey hooded top. He listened in silence to opening comments from the prosecution and his attorney before media were ejected.
The authorities asked the media not to name the boy because of his age.
He has been in custody since January after breaking the terms of his bail. Because of his young age he had faced up to five years behind bars.
According to the Austrian interior ministry, more than 200 have done so, including some women and minors.