According to the research firm, Google Android handsets accounted for 27 percent of all smartphones sold in the US in Q1, while iPhones made up 23 percent.
"While the iPhone has been the headline grabber over the last few years in the smartphone market, Google's Android OS has shown the most significant expansion in market share among current subscribers," the company said in a blog.
Neilsen also said that smartphones account for a quarter of the mobile phone market in the US. The research firm predicts smartphones will overtake feature phones as the most popular type of mobile phone by the end of 2011.
When it comes to handset loyalty, Apple's smartphone wins hands down. Eighty-nine percent of iPhone users say they'll purchase another iPhone for their next mobile device, while just six percent claim they want a handset that runs Google Android.
However, 21 percent of users with a phone currently running Google Android said they want an iPhone as their next mobile phone.