The Android handset passed the milestone less than six months after hitting the 10 million sales mark, making it the bestselling Galaxy device to date.
Network-specific releases in the US and the launch of the Galaxy S2 White in the UK were attributed to the recent surge in demand. The phone debuted in April last year, meaning that it has been shifting an average of 2 million units per month.
The Samsung Galaxy S2 has a 1.2GHz dual-core processor, along with an eight-megapixel rear mounted camera, 1GB of RAM and Android's Gingerbread mobile operating system. An upgrade to Android 4.0 Ice Cream Sandwich is rumoured for next month.
Samsung is expected to unveil the Samsung Galaxy S3 later this year, though the company has confirmed that it will not be shown at this month's Mobile World Congress in Barcelona.
Earlier this week, the firm announced plans to launch the Galaxy S Blaze 4G smartphone in North America.
Watch an advertisement for the Samsung Galaxy S2 below: