The report by market tracker IDC showed a 12.6 percent year-over-year decline in tablet sales in the July-September quarter, with 48.7 million devices shipped.
IDC analysts said tablet owners are not replacing the devices as frequently as in the past. And some are shifting to large smartphones, or "phablets," or to lightweight PCs, some of which have detachable keyboards.
"We continue to get feedback that tablet users are holding onto devices upwards of four years," said IDC analyst Ryan Reith.
"We believe the traditional slate tablet has a place in the personal computing world. However, as the smartphone installed base continues to grow and the devices get bigger and more capable, the need for smaller form factor slate tablets becomes less clear. With shipment volumes slowing over four consecutive quarters, the market appears to be in transition."
South Korea's Samsung held the number two spot with a 16.5 percent market share, even as the number of units sold slid 17 percent to eight million.
IDC said the global installed base of tablets at the end of last year was 581.9 million up 36 percent from 2013 but slowing.
"With mature markets like North America, Western Europe, and Asia/Pacific well past 100 million active tablets per region, the opportunities for growth are getting fewer," the IDC report said.
Apple is seeking to carve out a new niche in the tablet market with the iPad Pro, which has a detachable keyboard and is expected to go on sale next month, while Samsung has a similar device called the Tab S2.