Phenom II is a family of AMD multi-core 45 nm central processing units, succeeding the original Phenom. The Socket AM2+ version of Phenom II was released in December 2008, while Socket AM3 versions with DDR3 support, along with an initial batch of triple- and quad-core processors were released on February 9, 2009. Dual-processor systems will require Socket F+ for the Quad FX platform.

The Phenom II is the processor component of AMD's Dragon Platform, which also includes the 700 series chipset and Radeon HD 4800 series graphics. The Phenom II triples the size of the shared L3 cache to 6MB, from the Phenom's 2MB, leading to benchmark performance gains as high as 30%. Another change from the original Phenom is that Cool 'n Quiet is now applied to the processor as a whole, rather than on a per-core basis.

This was done in order to address the mishandling of threads by Windows Vista, which can cause single-threaded applications to run on a core that is idling at half-speed. Socket AM3 versions of the Phenom II are backwards-compatible with Socket AM2+, though this is contingent on motherboard manufacturers supplying BIOS updates. In addition to the Phenom II's pin compatibility, the AM3 memory controller supports both DDR2 and DDR3 memory (up to DDR3-1333), allowing existing AM2+ users to upgrade their CPU without changing the motherboard or memory.

However, similar to the way the original Phenom handled DDR2-1066, current Phenom II platforms limit the usage of DDR3-1333 to one DIMM per channel; otherwise, the DIMMs are underclocked to DDR3-1066. AMD claims that this behavior is due to the BIOS, not the memory controller, and plans to address it with a BIOS update. The dual-spec memory controller also gives motherboard manufacturers and system builders the option of pairing AM3 with DDR2 to reduce overall system cost, as compared to competing chips from Intel, which require DDR3.

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