Dickie Chang, senior market analyst at research firm IDC, said Apple supplied 28 percent of tablet computers in China during the April-June quarter, down from 49 percent dominance a year earlier.
The iPad maker was still the biggest tablet supplier in China, its key growth engine, but its momentum has slowed.
Apple sold 1.48 million iPads in the period, up 28 percent over a year earlier, but sales of Galaxy tablets made by Samsung Electronics quadrupled to 571,000 units. Samsung claimed 11 percent tablet market share, up from 6 percent.
Samsung, the world's largest smartphone maker, said earlier this year that it aims to double its annual sales of tablet computers, hoping to close the gap with Apple.
China's Lenovo and Taiwan's ASUS and Acer also had a surge in their market share. Chang said these companies benefited from offering cheaper tablets and from consumer familiarity with the Android operating system that is used in many smartphone models.
"If Apple cuts the price of previous generations of product like it did in phones, then more consumers would love to buy Apple's iPad," he said in an email.
China is a key market for consumer technology companies as growth in sales of smartphones and tablets slows in developed countries.
Apple CEO Tim Cook told the official Xinhua News Agency in January that China will become the company's biggest market.