It marks a victory for Coca-Cola Co. as its sodas now hold the top two spots, beating out its longtime rival PepsiCo Inc.
Diet Coke's rise reflects a long-term trend toward diet sodas. Ten years ago, only two of the top 10 were diet sodas. Now, four are on the list: the diet versions of Coke, Pepsi, Mountain Dew and Dr Pepper.
Coca-Cola sold nearly 927 million cases of its diet soda in 2010, to Pepsi's 892 million, a report by trade publication Beverage Digest released Thursday said.
Regular Coke remains far and away the most popular soda, selling 1.6 billion cases.
Overall, U.S. soft-drink sales have fallen for six straight years as consumers switched to healthier alternatives such as juices and tea and cut back on spending in the recession.
While both Diet Coke and Pepsi sold less soda in 2010, the decline was more pronounced for Pepsi.
The downward trend in U.S. soda sales puts more pressure on the beverage companies to compete.
Coca-Cola has pumped up its traditional advertising, including online ads. PepsiCo, which has lost market share in recent years, maintained some traditional ads but also steered dollars toward it Pepsi Refresh Project, an online donation program meant to build brand awareness.
Though the Refresh Project has proven popular, some have questioned whether it actually drives soda sales.
Coca-Cola Co. sold 0.5 percent less soda in 2010. For PepsiCo, the figure fell 2.6 percent.