The California-based firm said the 'intruder' might have accessed personal information like developers' names, mailing addresses, and email addresses.
The computer and software major has closed down the site, where programmers write applications for Apple products like iPhones, Macs, etc, on July 18 for overhauling and updation.
"Last Thursday, an intruder attempted to secure personal information of our registered developers from our developer website," Apple said in a statement on its developer's website.
"Sensitive personal information was encrypted and cannot be accessed, however, we have not been able to rule out the possibility that some developers' names, mailing addresses, and/or email addresses may have been accessed.
"In the spirit of transparency, we want to inform you of the issue. We took the site down immediately on Thursday and have been working around the clock since then," the statement further added.
Apple said it is overhauling the system to prevent a similar incident from occurring in future.
"In order to prevent a security threat like this from happening again, we're completely overhauling our developer systems, updating our server software, and rebuilding our entire database.
"We apologise for the significant inconvenience that our downtime has caused you and we expect to have the developer website up again soon," it said.