Bires replaces Brad Keselowski, who will join Penske Racing's Sprint Cup program in 2010.
The 25-year-old Bires has made 66 Nationwide starts since 2006, with 11 Top 10 finishes, including a career-best fourth-place finish at Nashville in April while driving for Kevin Harvick Inc.
Earnhardt called Bires a "talented" driver and is eager to give the Wisconsin native an opportunity to showcase himself.
JR Motorsports fields both the No. 88 and the No. 5 car in the Nationwide Series. There has been no decision on which car Bires will drive next year.