The two firms said they would produce live auctions at Sotheby's headquarters in New York and create a special Web portal that allows for "real-time bidding from anywhere around the world."
The companies will also "explore themed and time-based sales, as well as live auctions from Sotheby's other global salesrooms."
"The growth of the art market, new generation technology and our shared strengths make this the right time for this exciting new online opportunity," said Bruno Vinciguerra, Sotheby's chief operating officer.
"We are joining with eBay to make our sales more accessible to the broadest possible audience around the world."
Devin Wenig, president of eBay Marketplaces, said the partnership "is a significant milestone in our efforts to expand the live auction market."
"Sotheby?s is one of the most respected names in the world," Wenig added.
"When you combine its inventory with eBay's technology platform and global reach, we can give people access to the world's finest, most inspiring items -- anytime, anywhere and from any device. That is an experience we believe our customers will love."
California-based eBay has some 145 million customers in 190 countries, and says 36 million purchased some $8 billion in collectibles from the site in 2013.
Sotheby's says online bidders accounted for about 17 percent of the lots offered in 2013.
According to surveys cited by the two firms, the global art market is currently estimated around $65 billion and online art sales could reach $13 billion by 2020.