Users on Reddit noticed that they could download the update, despite the lack of any announcement from AT&T.
AT&T has since confirmed to TechRadar that the update is live.
"We can confirm that JellyBean update for GSIII is now available," an AT&T spokesperson told TechRadar. "Customers can get the 4.1 update via Kies."
The Jelly Bean 4.1.1 update is only available for Galaxy S3 smartphones through Samsung's Kies software on PC or Mac computers. No over-the-air release for the firmware update will be available for AT&T users.
Upgrading to Android Jelly Bean 4.1.1 offers Galaxy S3 users improvements to Wi-Fi calling along with keyboard and camera enhancements.
In addition, Jelly Bean offers a myriad of refinements over Ice Cream Sandwich, including improved notifications, more customizable widgets, and faster performance overall.
Jelly Bean still rolling out
In October Sprint was the first carrier to offer Jelly Bean 4.1.1 on the Samsung Galaxy S3, followed by T-Mobile in November. Both carriers offered the update through either the Kies software or as an over-the-air download.
No explanation was given as to why AT&T subscribers will not have an over-the-air download option.
Verizon and US Cellular are now lagging behind, as the competing carriers have yet to announce Jelly Bean availability for their own Galaxy S3 smartphones.
Then again, it took a leak through the Kies software for AT&T to acknowledge the update, so it might not hurt for Verizon and US Cellular users to check the software for a surprise treat.
Get daily insight, inspiration and deals in your inbox
Get the hottest deals available in your inbox plus news, reviews, opinion, analysis and more from the TechRadar team.