The new iMacs all have Intel quad-core processors, which had been available as an upgrade to the standard dual-core processors of the previous generation.
Apple said the new models are up to 70 percent faster.
The new 21.5-inch iMac has a single Thunderbolt port, which can carry data at speeds 20 times faster than most current USB ports.
It can also connect to additional monitors. The 27-inch iMac has two Thunderbolt ports.
The 21.5-inch iMac starts at $1,199, the larger model at $1,699.
Apple last revamped its iMacs in July 2010.