The decision in the long-running battle to get the data rejects arguments by the province's Community Safety Ministry that the information could identify individual offenders.
The province's Information and Privacy Commissioner had previously ordered the records be disclosed to the unnamed journalist.
Last year, the Divisional Court dismissed the ministry’s application for a judicial review of the commissioner's decision.
The ministry had said the information — requested through Freedom of Information laws — could reasonably lead to an expectation of harm, facilitate the commission of an illegal act, or hamper the control of crime.
However, in ordering release of the data in 2009, the privacy commissioner found the ministry's concerns baseless.
"The commissioner had already determined that it was not reasonable to expect that any individual could be identified from the requested information," a Divisional Court panel ruled last June.
"Consequently, he concluded that the ministry had failed to provide a reasonable basis for believing that endangerment would result from disclosure."
In upholding that finding, the Ontario Court of Appeal said it agreed the commissioner's conclusion was supported by the evidence and was "reasonable."
The ministry provided no evidence that providing the first three digits of the postal codes could be used to locate convicted sex offenders within communities, the appellate court ruled.
Sex offenders are required by law to notify the Ontario registry if they change addresses. Information in the registry is normally off-limits to anyone other than police involved in an investigation.
The ministry worried that disclosure of the information could prompt offenders to go underground out of fear of vigilantism.
On average, more than 24,000 people live in the forward sortation area identified by the first three digits of a postal code.
The Appeal Court said there was no evidence providing the requested information could "engender an offender's subjective heightened perception of this possibility, thereby adversely affecting compliance with the sex offender registry."