The IDC report said sales of tablets including newly introduced convertible PCs totalled 50.4 million units in the first quarter of 2014.
That was just 3.9% higher than the same period a year earlier, and down 35.7% from the busy holiday season that included the fourth quarter of 2013.
IDC said that the market is suffering from saturation, and that many people are using large-screen smartphones or "phablets" instead of tablets.
The introduction of new "2-in-1" devices such as the Microsoft Surface, which aims to serve as a PC and a tablet, failed to gain traction.
"The rise of large-screen phones and consumers who are holding on to their existing tablets for ever longer periods of time were both contributing factors to a weaker-than-anticipated quarter for tablets and 2-in-1s," said Tom Mainelli, an IDC analyst.
"In addition, commercial growth has not been robust enough to offset the slowing of consumer shipments."
Tablet sales have been surging and grew roughly 50% in 2013, but that pace appears to be slowing.
IDC said Apple remained the largest tablet vendor with 16.4 million iPads sold in the first quarter. But its market share slid to 32.5% from 40.2% in early 2013.
Samsung captured 22.3% of the market with 11.2 million tablets sold. Its market share has fallen from 32% a year ago.
Amazon saw a sharp decline in sales of Kindle Fire tablets - down 47% from a year ago to one million units. That put Amazon in fifth place with a 1.9% share, according to IDC.
Taiwan's Asus was the number three seller with a 5% share, and China's Lenovo was fourth at 4.1%, IDC found.