In August, the Gothenburg District Court acquitted the boy, who was put on trial for having downloaded at least 24 films from the internet and then sharing them with others.
As he was 15 at the time of the initial trial, he is considered to be the youngest person ever to be put on trial in Sweden for filesharing crimes.
At the time of the boy's acquittal, prosecutors called the lower court's ruling “absurd” and vowed to appeal.
On Wednesday, the Court of Appeal for Western Sweden reversed the district court's ruling, and convicted the teenager for downloading copyrighted material.
The boy was reported to police by his school principal after the school's IT department discovered the downloaded films on his computer.
They made the find after having identified the boy's computer as the source of a virus that had infected the school's computer system.
While he admitted to downloading the films, the 16-year-old claimed he was unaware that his actions were criminal.
According to the appeals court, the boy must have realized it was “possible that his downloading covered copyright protected material that he couldn't legally download”.
“When he, despite this, downloaded the films without finding out the particulars, he behaved with serious recklessness,” the court wrote in its ruling, the Göteborgs-Posten (GP) daily reported.
“In this respect, he is guilty of committing a crime against the copyright law.”
The court ordered the boy to pay a fine of 1,500 kronor ($220).
As the offence was limited and not seen to be a part of any larger organized filesharing operation, the court's sentence was relatively lenient.
Nevertheless, the Pirate Party, a political party which wants to see reforms of copyright laws, condemned the guilty verdict.
“The appeals court's ruling means that hundreds of thousands of young Swedes are at risk of being dragged into court,” party leader Anna Troberg said in a statement.
“This is not right. Today's copyright laws are wrong in that they criminalize an entire generation of young people and throw a spanner in the works for new creators of culture that see technology's possibilities.”