Robert Griffin III is not only impressing on the field, he's also making an impression off it. The Washington Redskins rookie quarterback's No. 10 jersey has sold more than any other player's in a single season since the NFL started keeping track six years ago, league spokeswoman Joanna Hunter told ESPN.com.

The metric is based on sales of jerseys on the league's official Internet store, NFLshop.com.

No. 10 is No. 1 this season in NFL jersey sales. Justin K. Aller/Getty Images

The NFL will not provide specific sales numbers but did say that Griffin moved into the top spot by overtaking Brett Favre, who previously held the record for most jerseys sold in a year twice -- with the New York Jets in 2008 and the Minnesota Vikings in 2009.

In the past couple of weeks, Peyton Manning's Denver Broncos jersey has become the fourth-highest seller, surpassing sales of Dallas Cowboys quarterback Tony Romo in 2007.

What makes Griffin's accomplishment even more impressive is that the NFL's fiscal year runs from April 1 to March 31, meaning that he has the league's highest-selling jersey with 3½ months to spare.

Playing a small role in sales has to be that the NFL changed its official apparel manufacturer this year from Reebok to Nike, which signed a five-year deal.

For December, Redskins merchandise sales on Fanatics.com -- one of the nation's largest online retailers -- is up 252 percent compared to the same dates last year, which is the highest percentage increase among NFL teams this month.

Fanatics spokesman Meier Raivich said that an impressive 87 percent of Redskins merchandise the company has sold since the start of the season has been purchased outside the Washington, D.C., Maryland or Virginia areas.

Everything related to Griffin is selling, including the line of socks that adidas started producing for the quarterback after signing him to an endorsement deal. Adidas spokesman Michael Ehrlich says the company has sold out of the socks, which sell at the premium price point of $20 per pair.