Google had 528 million unique visitors in March, up 5 percent from the previous month, according to comScore. Microsoft had 527 million visitors during the same month, up 3.7 percent.
Popular in the United States, Google is even more of a powerhouse in many European countries.
In a statement, Google said: "Our goal has always been to provide the best online experience for our users. We build products based on user needs and input, which is part of what makes Google unique and results in a great online experience."
Microsoft declined to comment.