Apple's share of the global tablet PC market stood at 50.4 percent as of the third quarter of this year, down from 60 percent a year ago, market researcher IDC said Monday.
Rival Samsung Electronics' share more than doubled from 7 percent to 18.4 percent over the same period. Amazon's "Kindle Fire" accounted for 9 percent.
The entire global tablet PC market is estimated at 28 million units, up more than 50 percent from 2011.
IDC said the adoption of Google's Android OS on a wider range of smartphones and tablet PCs is hitting Apple. Samsung already sells more than twice as many smartphones as Apple.
But some analysts say sales of the iPad dropped temporarily because consumers waited for the release of the iPad Mini. Apple said sales of the iPad Mini totaled 3 million units just three days of its release last Friday and expect the market share of its tablet PCs to increase in the fourth quarter.