Its share of searches in the US. reached 10.7 percent last month, up 0.4 percentage points from November, comScore said.
Google is still attracting the majority of searches, with a 65.7 percent market share in December, up 0.1 percentage points from November, comScore said.
Microsoft's growth came at the expense of its partner Yahoo, as well as Ask and Aol. Yahoo's share slipped slightly from 17.5 percent in November to 17.3 percent in December. Aol also lost share, slipping from 2.8 percent in November to 2.6 percent in December.
Since it launched Bing in June, Microsoft has steadily if slowly been gaining ground. The company has added new features over that time, perhaps contributing to the growth.
In mid-November, Bing results began relying on computational information from Wolfram Alpha, allowing people to type complicated math equations or ask Bing to display a graph plotting an equation. The update also allows people to type in the name of a food item and receive nutritional information.