Adam Mudd admitted creating malware in 2013 which was used to carry out 1.7 million cyber attacks.
The 20-year-old of Toms Lane in Kings Langley was jailed for two years at the Old Bailey in April.
But on Thursday, judges at the Court of Appeal found the original punishment had not given enough credit for his guilty plea.
Mudd, who made more than Â£380,000 worth of US dollars and Bitcoins from victims including gaming websites Minecraft and Xbox Live, had his sentence reduced by three months.
A computer hacker who masterminded global online attacks from his bedroom has had his prison sentence reduced.