A BP spokesman said in an e-mail to The Associated Press that the 50-foot, 300-ton device was detached from the wellhead at 1:20 p.m CDT.
The device was slowly being lifted to the surface and wouldn't likely reach the top until sometime Saturday.
Earlier in the day, a vessel had latched onto the equipment to raise it from a mile beneath the sea.
Undersea video showed the device suspended in the water. A crane on the Helix Q4000 was being used for the task.
The blowout preventer is considered a key piece of evidence in the spill investigation. Investigators will examine it and hope to gain insight into why the device failed to prevent the spill.
The April 20 explosion aboard the Deepwater Horizon killed 11 workers and led to 206 million gallons of oil spewing from BP's undersea well.
Investigators know the explosion was triggered by a bubble of methane gas that escaped from the well and shot up the drill column, expanding quickly as it burst through several seals and barriers before igniting.
But they don't know exactly how or why the gas escaped. And they don't know why the blowout preventer didn't seal the well pipe at the sea bottom after the eruption, as it was supposed to. While the device didn't close - or may have closed partially - hearings have produced no clear picture of why it didn't plug the well.
Lawyers will be watching closely, as hundreds of lawsuits have been filed over the oil spill. Future liabilities faced by a number of corporations could be riding on what the analysis of the blowout preventer shows.
The raising of the blowout preventer followed Thursday's removal of a temporary cap that stopped oil from gushing into the Gulf in mid-July. No more oil was expected to leak into the sea, but crews were standing by with collection vessels just in case.
The government wants to replace the failed blowout preventer first to deal with any pressure that is caused when a relief well BP has been drilling intersects the blown-out well.
Once that intersection occurs sometime after Labor Day, BP is expected to use mud and cement to plug the blown-out well for good from the bottom.