Julius Kivimaki was found guilty of 50,700 "instances of aggravated computer break-ins".
Court documents state that his attacks affected Harvard University and MIT among others, and involved hijacking emails, blocking traffic to websites and the theft of credit card details.
Despite the severity of the crimes, the 17-year-old has not been jailed.
Instead, the District Court of Espoo sentenced the youth - who had used the nickname Zeekill - to a two-year suspended prison sentence.
It also confiscated his PC and ordered him to handover ?6,588 (?4,725) worth of property obtained through his crimes.
Judge Wilhelm Norrmann noted that Kivimaki had only been 15 and 16 when he carried out the crimes in 2012 and 2013.
