Online sales were 23% higher in the three months to December.
However, that was less than half the growth recorded in the previous quarter and lower than in the same period of 2016.
Shares in the world’s largest retailer fell just over 10% to $94.11.
The retailer said its online revenues totalled $11.5bn last year, but it lost money on those sales. Chief executive Doug McMillon said e-commerce losses would be “about the same” for 2018.
He told analysts the company was making progress in its efforts to compete with Amazon and other competitors.
“We’re confident in our strategy to transform the company,” Mr McMillon said. “It’s really about providing more convenience to customers.”
It plans to spend more on Walmart.com and cut back on marketing for its Jet.com site, which aimed at younger and better off shoppers. Walmart paid $3.3bn to buy Jet.com in 2016.
While sales at Walmart’s US stores rose by a better than expected 2.6% in the fourth quarter, net profit sank 42% to $2.2bn (£1.6bn).